This AU is… complicated? I'd say it's endgame Mihawk/Shanks/Benn triad in a very non-traditional relationships way. For example, by the time Mihawk semi-joins them and turns down the Warlord position, even Shanks and Mihawk don't have a concrete romantic relationship, and theirs is still the closest to that, haha. I guess it's best classified as:

Shanks and Mihawk: close relationship bordering on romantic; there's mutual interest, but their definitions of romantic also don't match "standard" definitions of romance, so it doesn't always read as that to others. Rivals/partners with a romantic flair?? Since Mihawk turning down the Warlord position and choosing to join them comes with a good chunk of other upheaval, there's a conscious decision to not really label anything, but they're on the same page of being mutually interested.

Shanks and Benn: hellooooo complex intertwined captain-first mate relationships. The fun duality of being both incredibly simple and incredibly complex. I'm team "many working captain-first mate relationships basically become life partnerships", and they're probably the prime example of that. Inconveniently for tagging purposes, it's smack dab in the middle of "&" and "/" relationships for AO3, so… eh, the vibes are vague. The details past the fact that they're basically each other's most trusted person stop mattering at a certain point.

Benn and Mihawk: Complicated, haha. By far the side of the triad with the most development to be had; right now, it's more like they're separate sides of a V polycule, rather than a triad, but the foundation's there. An odd, unexpected friendship turned an even more unexpected camaraderie as the years go by and Mihawk's around more and more. Absolutely no one outside of them (and occasionally Shanks) can really parse out what's going on here. Again, just over the line of what I'd tag as "&", and develops veeeeeery slowly over the years in its own undefined way.

…That ended up as a whole ass wall of words, haha. Shorter summary if you're looking for general relationship shifts:

Mihawk sails with the Red Hair Pirates (~6 years after the RHP start): only Shanks/Mihawk are semi/almost-romantic

Sailing time until Dawn Island: Shanks/Mihawk slowly get their shit together; Shanks/Benn inch toward maybe another realization or two, Benn/Mihawk "two introvert cats" bond starts really kicking in

Post-Dawn Island: Shanks/Benn get their shit together (thank you, realizations about mortality)

The decade-ish between Dawn Island and current canon: It's a polycule between an Emperor, an Emperor's first mate, and the World's Greatest Swordsman. Who cares if it makes sense to anyone else; it works for them, lol.

Observing how things often devolved as the night went on aboard the Red Force was always a good time, one that Benn had become very acquainted with over the years. It certainly wasn't that he never joined in, but sometimes, there was just as much amusement to be had in watching the chaos unfold instead of being at the center of it. His keen eyes caught details often missed by others — such as noticing a particular swordsman appearing, coming into view as he deftly traversed the crowd of increasingly drunken pirates.

Mihawk had boarded the Red Force once more earlier that evening, his raft appearing on the horizon after multiple weeks of being away, just as the crew began to settle across the deck after dinner. Benn had half-expected him to simply disappear into the privacy of Shanks', or his own, cabin, regardless if either of them were there. He certainly joined — or at least, joined in as much as he ever would — in the near nightly festivities more than he would have years ago, but rarely so after long journeys.

Despite that, he was approaching Benn at his place on the edges of the gathering. While he still donned his hat and coat, Yoru wasn't attached to his back, a once rare sight that had become slowly more common over the years. With a tilt of his head, he perched himself atop the other half of the crate Benn was sitting upon.

It was odd seeing Mihawk without a wine glass in his hand at such a gathering, something it seemed he had made no move to grab. They had long since kept some of his preferred wines available — some, because it'd destroy Benn's carefully crafted budget if they got the rarer of his favorite vintages — and even some half-decent glassware.

“Don’t want anything?” Benn asked, gesturing toward the open crates of various bottles stacked off to the side of the deck.

“Mm.”

Mihawk's curt response pushed Benn to take a closer look at the other man. The faint bags under his eyes and the odd set to his shoulders were undeniable. Those who didn’t know him well wouldn’t see anything but a menacing figure staring down the deck. To Benn, though, it was obvious how far exhaustion had seeped into the swordsman. Mihawk never slept that soundly when he was off sailing alone. Few people sailed alone for so long for a reason; keeping up navigation, watch, and every other duty left room for barely more than catnaps. Mihawk's observation haki and strength combined certainly left him in a more secure position than most, but wear on the mind and body was inevitable.

That wasn't even accounting for Mihawk's naturally odd sleep patterns. They was outright baffling at the best of times, even aboard their ship, and the rumors of him being a vampire were only fanned by his insomniac tendencies. More than once, Benn had ran into him in the middle of the night, more awake and alert than even the crew on night watch.

Shanks’ presence at least did something good in getting Mihawk to sleep more regularly. Really, they were both much more manageable the next day if they had snuck off to curl up in Shanks' cabin the night before.

But it had been weeks since they had even had the opportunity to do so, and it showed.

"Surprised you're willing to put up with this on your first night back."

Mihawk huffed, a dry, exasperated noise that spoke volumes on its own. “The noise would be as inescapable as always, a deck between or not."

Benn snorted at that. Indeed, there was nothing stopping a party aboard the Red Force. Even an attack would be nothing more than a brief interruption — hell, if it wasn’t really a threat, he doubted most of the boys would even put down their drinks.

Benn let any further response drift away in favor of a comfortable quiet, one that Benn was content to sit in. Their time together was often quiet in the first place, and quiet company was good company. Company that not just Shanks had missed over the last few weeks.

It wasn't long before Benn felt the lightest of pressures pressing against his side, warmth spreading across his arm. Out of the corner of his eye, the brim of the swordsman’s hat tipped down ever so slightly, shading his face from view. Something warm and bright stirred in his chest as he realized that Mihawk had found somewhere to sleep after all.

His arm snaked out from its pinned position between them, coming to rest at the exposed strip of skin on Mihawk’s hip. It was barely a hold, much looser than Shanks’ own were, but it kept him snug against his side all the same. A nearly imperceptible sigh slipped from the other man’s lips, and Beck felt the last tendrils of tension leave the other man’s form as he sunk into the contact. (If Mihawk was willing to allow physical contact — in initiating it, even in front of the crew — he could meet him in the middle.)

“Wasn’t planning on getting up anyways,” he murmured, lighthearted, fondness openly seeping beneath his low tones. "Glad to have you back."

The raucous sounds of the deck carried to his ears, and he let them wash over him, lazily smiling at their antics. Whatever drinking game they had been playing had devolved beyond comprehension while he wasn’t looking — not that it was anything more than a flimsy excuse in the first place. It was easy to spot his captain, even right in the midst of the crowd, between the shock of red hair and the familiar laughter and shouts echoing across the deck. It didn't take long for his captain to turn back towards him, no doubt having felt his eyes on him with his usual keen sensing.

What he hadn’t felt, evidently, was the relaxing of Mihawk’s haki as he had fallen asleep. Benn saw the half-formed call on his lips fail to make a sound, the man frozen as he stared blatantly at the sight before him.

It was nearly comical at how transfixed Shanks was. One would have thought Shanks had never seen the other man before; it was a miracle his beer bottle was still intact in his hand and not spilled or dropped. The other pirates around him paid him no mind, too drunk to notice his sudden pause — or too used to his antics to care.

Shanks would make his way over before the night was through, Benn was sure. Keeping him away from his swordsman was an impossibility, but for the time being, he was getting more than his fair share of amusement from where he was. His smile, both fond and impish, said enough by itself as he stared intently at them and raised an eyebrow teasingly.

Even though a vehement, crude gesture would have been warranted in return, Benn didn’t dare move beyond a scowl and a rolling of his eyes back at his captain as he turned away. Mihawk’s breathing had just begun to even out, and between the man’s observation haki and his near-constant raised guard, it'd take even less than a jostle to awaken him. Considering the man’s… temperamental reactions to being awoken too early, even when well-rested, he had no interest in facing that.

(And if Mihawk's slumbering presence at his side meant sleeping sitting up on the deck for the night... some things were worth indulging, no matter the aches and pains that were sure follow by morning.)