I was debating how to do this but, I think I'll start with Beomgyu.

I largely agree with your assessment of him. Not that he's not batshit insane and built for variety but, it definitely feels part of his idol persona to me. In a somewhat similar way to Yeonjun, I think he's way more sentimental, delicate and soft than people realise. Which makes the juxtaposition all the more fun and inticing. Because unlike Yeonjun, while I do think a lot of this is performance from Beomgyu, 1. He commits and 2. There is truth to it. Or maybe it feels like truth because he commits so hard.

It takes a lot, I think, for Beomgyu to break and/or cringe at himself and his actions. He also does the typical dude kpop idol thing where part of the appeal for him seems to be making his members cringe (usually YeonBin because Tyunning (Taehyun and Kai) have stronger resolves/thresholds. Though you could also argue the age dynamics of it all with Beomgyu playing the annoying younger brother to YeonBin's long suffering and deeply tired older brothers and Tyunning being like there goes weirdo older brother Beomgyu again).

I think the appeals of Beomgyu's brand of being a freak is how far he's willing to go and, the gap that exists between screaming, barking, occasionally grabbing his dick on stage Beomgyu and the Beomgyu that speaks soooo softly and sweetly and writes songs (rather sincerely at least coming from a non-native Korean speaker) about wanting to be a comfort for his fans. I know this is a selling point for a lot of idols but, I think he's one of the few that do it quite well and it doesn't come across forced to me.

I also think it's fascinating that he's such a menace but, he's also so deeply modest. It's very cute to me actually. I think a lot of people think unhingedness translates to the stage as well but, that's not really the case for Beomgyu. He covers up a lot. He rarely gets sexual. The only person more pg13 than him in TxT is Kai (who seems to be stepping away from this recently. Interesting. Can you tell I have a lot of feelings and thoughts about Kai and this topic? Which is terrible of me. I'm being an awful girlfriend right now). The literal baby of the group. He'll grab his dick and bite his lip and heavily imply he could fold you like a pretzel when needed but, usually his brand of fanservice tends to be quite sweet, actually. Fascinated by him. Love him. Desire him carnally. Need to study him like a bug.

I was going to preface the Taehyun section by saying I have a lot to say about him but, that's before I realised I typed an entire novel about Beomgyu 💀

Taehyun is the inverse of Beomgyu. He appears sweet and soft and cute. To me, he's the cutest looking member of TxT. The fact that his voice is what it is helps to make people assume he's nothing but, a sweetiepie too, I think. Also, stereotypes people have about main vocalists generally. Him being the shortest adds to the all this too I think. Not to mention his affinity for making cute noises that make me want to weep at my big age of 25 and his more calm, lowkey personality, even in variety settings.

Taehyun uses this misunderstanding of who he is to his advantage. I think where Beomgyu plays into the gap to give us a best of both worlds experience, Taehyun uses it in a way that is meant to jar us. Startle us. Catch us off-guard. And I say that because of the ways he tends to go about exploiting the differences in his appearance/idol presentation.

As you've said, Taehyun has a more quiet insanity about him. Quite Seonghwa-like actually Free me help me get me out of here. He'll some wild shit in a tone that sounds so normal that you'll have to take a second to make sure you read the subtitles correctly. One of my all time favourite Taehyun clips is when he was playing a game with Beomgyu and Kai I believe and Kai kept doing some fuck shit and Taehyun threatened to break his hand if he did it again. All blasé and with his disarmingly cute face. I think he even smiled afterwards. Just an insane man. I love him. Also, Taehyun has a knack for making his bits not sound like bits. He's very good at sounding serious and/or playing things off. Which I think he knows adds to his appeal in general but, particularly when he says or does something fucking crazy.

Also, Taehyun is an exhibitionist. He is the exhibitionist in TxT. He's also the most comfortable being sexual. He's perverse in a very typical way. I'm particularly thinking about how he loves to show off his body in a way he knows will get his fans hot and bothered. He loves that shit. He lives for it. I'm not sure how familiar you are with his posting habits but, Taehyun loves to post photos of his back or torso or a front facing selfie where he's shirtless and it cuts off before you can see below his collarbone and caption it with some shit like 'oh, I love the weather today.' He loves being a little shit and he loves the reactions he gets for it. I think out of everyone in the group, he enjoys being the object of his fans' lust and enjoys that power that gives him the most. He's also been very vocal about wanting more horny songs (of course he didn't say it like that but, we all know what he means) and he's very at home performing the horny songs. He's very comfortable selling the I'll put you through the mattress fantasy to his fans. Probably because it's not a fantasy and he's very capable and willing to do just that.

So people see this dude who is so sweet looking and sounding and expect his actions to reflect that only for him to send the idol equivalent of a 'you up?' text on Weverse, be very comfortable talking about inflicting pain on people and use his very pretty voice and face and body to perform how he's more than happy to eat you out and dick down (we should talk about Upside Down Kiss more. What a fucking crazy song).

God, he really is the Seonghwa brand of being a sick freak. Help me.

I think him being a gymrat adds to the whole exhibitionist thing and he's typically regarded as the strongest TxT member. I'm sure that feeds into his whole condescending I'm not as sweet and nice as you think I am and I am very comfortable throwing my members around and I can very easily throw you around too thing. Maybe that's projection on my part though, who knows.