I can't fucking sleep, so here's that Bro analysis ramble y'all asked for.
Bro is, hands down, the king of imposters. In the sense that this dude is, older Dirk once removed, and it seems in his role and awareness of shit being Irrevocably Fucked he doubles down on his persona rather than drop the mask. He's a shit person with interests he engages in for the fact it's amusing and ironic, which in of itself is kind of a testament to the guy's mental state? Like, what kind of level of emotional hell do you need to be on to have interests that are almost certainly mostly ironic.
Masochism at it's finest.
We all know Dirk has enough thinly veiled self-hatred to power a fucking city, and this is multiplied in Bro. With the ASSUMED influence of Cal (if we're operating off that theory), his isolation, absolutely shit-sum fucking social skills and lack of genuine interests or outlets, the dude ends up becoming outwardly volatile and unpredictable, re: his shit behavior towards Dave.
It's those moments where the mask cracks and he displays the lack of "holding it together" he tries to act like he has the rest of the time.
If Dirk has a complex in terms of not looking out of control, Bro is fucking Napoleon.
That's where the parallels get drawn between Bro and Ult!Dirk. There's a similar theme: control your immediate surroundings so you can feel in control when you and your circumstances are, themselves, entirely out of your control. Give people the impression you're either entirely put together, or throw them off your trail, pick your poison. Bro seems to flipflop between the two.
We see this manifested in him controlling Dave from the beginning. It's a crude attempt of taking control of circumstances you don't have control of, and, additionally, didn't even want to. For a guy that has major control issues, he sure doesn't like responsibility.
He's jaded in ways Dirk isn't in the start of the comic, which is why he doesn't care. It can be assumed he started it all with good intent, but the dude had no fucking idea what interacting with people was supposed to look like, much like Dirk, and at some point when it would be made clear to him that conditioning your kid the way he does is Not Normal OR helpful, he's already too far gone to care. And... gestures to Cal. There's gotta be a connection, just saying, I think anyone would go batshit like that.
Anyway, this dude is a nightmare and is basically really similar to Ult!Dirk personality wise, except he doesn't seek villainousness to prove his own self-hatred a self fulfilling prophecy, he just doesn't see or care to check himself, but some people aren't ready for that conversation.