This is too sudden

This is pretty typical to occur with many long running series or authors that have many series they are working on.

Simply put.
They try to finish and wrap up the story as fast as possible. So the endings often become half assed and start having holes in the story and character developments.
It may be its just now in poor state....or she isn't sure what to do anymore.
But frankly it gives vibes that she is just pushing to end the story quickly now instead of giving it a proper graceful send off.

Jaekyung is still changing gradually. It was not really sudden to me.

Tbh it's a very slow burn redemption. Jaekyung says in his own head things have to change and has realized his feelings towards Dan but he hasn't taken any actual steps to make changes in how he treats Dan and communicates with him.

All the progress has literally just been in Jaekyungs head. It's all well and good to realize your mistakes but turning that into action and genuinely looking for forgiveness is a whole other thing. Instead of making changes Jaekyung mindlessly fell bad into old habits at the first opportunity. Bros got a long way to go.

I just feel like compared to the sheer number of chapters that he kept treating Dan horribly for (even if he was getting attached a little there), the realization in his head was way too quick imo. I knew this was gonna be a slowburn when I was 40 chapters in and horse face was still being a jerk. Doesn't it feel like he just snapped so quickly and now the comic is trying to be funny to show his redemption and Dan's reluctance.

yes i agree, sigh.

He faced the possibility of Dan's death. That will wake anyone up. He is now very phobic about that loss and is trying to protect Dan. So to be it's not fast. I was close to dying myself and watched my mom die. It changes u. So i am speaking from personal experience. The time he spent recovering and healing and watched Dan work and visit his grandma. He got a good rest. A much needed rest. It was about time he refocused. All his old ways stopped working. He could not hold the title no longer. It's time to change. I like the slow burn.