
Or, maybe someone with psychosis won't always act in a way you seem rational. While I don't have psychosis I relate to a lot of his symptoms and experiences, when you're in pain and can't trust what you see and hear giving up is not only common but a more "logical" choice than one might think. His actions are completely normal for his situation

No.
I hate this. I hate people like you. I hate how low your standards are. I hate how bad at reading comprehension you are.
Writing doesn't have to be bad. Characters don't have to be stupid. All of this can happen without giving your characters a lobotomy. It's just making them stupid is easy and brainless, it takes no effort.
And then people like you go "oh well sometimes people are just really stupid and just to really do things that are really stupid and it's okay that people are really stupid and that's realistic" but it's not. Most people aren't stupid. Most people are average.
We've seen this characters "psychosis" for 60 chapters and it's never made him stupid until now. It suddenly made him stupid to further the plot in the easiest way possible. We can see his thoughts and he's shown repeatedly that he is still very intelligent and capable. He has been a very consistent character but now we're seeing his thoughts and they're constantly contradicting each other.
The plot has also made it very clear that he likely doesn't have psychosis or mania at all and this is all a conspiracy created by Lister and that his medicine is designed to mimic the symptoms, that's why his symptoms have gotten so much better on this new medicine and he's getting his memories back. The "hallucinations" He's been seeing are just memories. He's not actually crazy and the new medicine he's on is making him more sane. We all know this... And yet the author has chosen to make him suddenly brain dead at the same time they're making him more same, lol.
It's just bad writing.

Its not bad writing.. its humanising Slyhan.. not dumbing him… he’s clearly very confused, sick and stressed and is making the “easy way out” decision at the moment.. Masterpieces don’t have high/low standard characters. They have characters that cannot be measured by standards. Thats how you know they are a masterpiece. It might be 60 chapters of the manhwa but it could be just the character building arc because there’s so many information that are not disclosed yet. I doubt the story will end with us getting the answer all at one (I’ll be disappointed if so) cause I want the answers to be told pieces by pieces and readers putting them together.
Also, a great story isn’t only about MC and ML getting together and having their happily ever after, its about how their story unfolds and how they got their ending. If not, there’s no point in having a plot at all.

Maybe spoilers: Realistic and logical internal conflict is impossible for someone on strong medications and suffering from mental health problems. His brain was basically destroyed by drugs and trauma and trying to fix that is gonna take a while. Even a less traumatized brain acts irrational idk what you expected from MC. Just wait, you see him improve a lot slowly once he goes away from lister and can let his brain slowly heal.

Wow okay, first of all drug induced psychosis is still psychosis. Why is it stupid to give up? He's been in this situation 2 years and he has done his best to protect himself despite having no one bat for him, Gjiord shows up and gives him affection but he can't remember him and is very aware he is no longer the person Gjiord fell in love with. The violence around him

Has escalated dramatically and he is exhausted, it's self preservation. We know that he once had a better life but he doesn't remember it and he only wants to preserve the peace he has. It's not what the we think would be best for him but it's in line with a protective mentality a mentally ill person would have. I assume you've never had hallucinations but I have and I feel like this is one the best representation I've seen.
Writers don't have to make their characters completely illogical in order to further the plot. You can spend time and actually think of realistic logical conflict, you don't have to do this.
Saying "I'll be a slave forever. Slaves know nothing and control nothing." In one breath and "I'll never let Lister have his way." In the next breath is just bad writing.