
I get that. I just can’t really grasp having any love left for someone who treated you like dirt and insulted you for the thing you’re most insecure about, then went after your friends for the same reasons. I’ve had crushes, but I imagine they’d all have immediately ended if they ever did anything like that to me.

I think near the end when Tasuku angry confessed to his crush in the classroom, the crush started to change. It's no 180 degree change, but he began to be more aware, reflect on his past behaviour and even question his own identity (see ch 20). He yelled at his very homophobic father about how he blabbed to Saki's father that Saki was lesbian (Not only because of the stigma attached to LGBTQ+ relationships but also because it's not for other people to announce another person's relationship without their permission of course). In ch 19 when his dad said "who lured you into becoming gay" he just said "no one". If this was the old Touma he probably would have made some homophobic remarks to try and defend himself. Later on in ch 20 he says "part of me thinks I likes guys but part of me thinks that can't be possible" and "I don't know how I can accept myself and ... I don't even know what I want to accept". When people defaced the wedding building in ch 22, Touma got upset and started crying (and said he was really looking forward to the wedding).
In terms of how can Tasuku still like a guy that is insulting to him and his friends, part of it may be that for a large part of his life he also had those feelings of internalized homophobia about himself so it might of not hurt him so much until he called Touma out in the classroom. It's kind of like when you think to yourself "wow I'm so dumb" and then when someone says "you're dumb" it hurts (never said it didn't), but you already think that way about yourself so you'd probably feel like "wow that was kind of rude but also not wrong". Of course it's not a good thing but I feel like Tasuku had this kind of mentality before he started to open up to the group (kind of like shooting an open wound vs opening a new one).
And as I said before, I think Touma redeems himself in the end or at least deserves a chance to redeem himself because he's reevaluating his past decisions and trying to change. Emotions, especially strong ones, are not things that are easy to let go of an move on. Even if Tasuku started to love Touma less after bad mouthing he and his friends, Touma began to change before all of that emotion (recall that the entire story only spans 2-3 months) disappeared and Tasuku was willing to give him a chance since he's properly considering his feelings. Keep in mind that before Touma got involved with the Cats Clutter, he and Tasuku were on not on bad terms and the "treated him like shit" part was only a few times during the very short period. That's my opinion anyways.
I totally understand if people disagree, but the biggest problem I had with this is that this guy continued to like his crush when he was being homophobic and cruel to him. I understand it was internalized homophobia, but how can you like someone who’s so shit to you and your friends?