Love is blind, for most people. I know some who, even after being rejected harshly, still pursued wooing their love interests. Most of them won their love successfully and are still happily and playfully enjoying their growing relationship, others gave up to look for someone they deserve better. In the older bro's and Yocchan's case, their is growth although quite gradual. Yeah, it's a common stereotype in BL but it's not far from real life too. At least the tsundere here isn't exaggeratedly abusive.
... hes not a tsundere. being an asshole does not equal tsundere. people need to stop justifying his horrible attitude and personality with 'hes a tsundere'. and this whole chapter, he pretty much punched him every chance he got. maybe hes acting like a tsundere now, but he 100% was just a straight up dick before.
Compared to how he was, yes. There's improvement on his part. Kenji is flawed from the very beginning and 'that' he developed overtime. He's an ass shaped by time and his many mishaps so one can't just expect him to change drastically. A drastic change is more temporary while a gradual one gives time for the person to reflect on many things first. I see here, on the latest chapter, that he has the capacity to change, and I as a viewer will stick around to see how he will grow to be better.
if this was real life, yoochan would have dipped already, and honestly, i would have preferred to see that. people are going to get fed up with his attitude at some point, but this is yaoi, and in the yaoi world, if you act like a piece of shit, it doesn't matter, youd still end up with a really handsome and good guy.