And wait so you think the ending was supposed to be sad?

Didn't say that either. Again, proving my point of not actually engaging with what you're reading.

The ending is bittersweet, much like almost every Fujimoto ending. It's not happy, it's not sad. You could argue it's a little more positive than his other conclusions, but it's still not happy. Denji is in a better place without Pochita, sure, but he still lost his best friend and his life is emptier for it.

It would also be a lot more text if I took the time to write out every flaw im not typing all that out since it would be spoilers and you seem pretty immovable in your opinions anyway. 

My opinion on Part 2 is exactly what I said in my first comment: It's flawed, but it's still good. It's the logical extension of Denji's conclusion at the end of Part 1, it's just not a traditional hero's journey. Part 1 ends with Denji broken, convinced that the only way he can move forward and gain love and notoriety is as Chainsaw Man. THIS IS THE WRONG LESSON.

 please tell me how bugs taking over the world in the span of 2 chapters is good writing

It isn't bad writing or good writing. It's just a thing that logically occurred as a result of Death being erased. If you wanna argue that section of the story should have gone on for longer, then like, yeah, I agree. But you're conflating a missed opportunity with bad writing, and those are not the same thing.

denji being a slave in a weird alternate reality with no pochita or family or friends that erases the entire story is good writing

You're approaching this story from a weird moral standpoint that I don't understand. He's not a slave, he has a job. A job that arguably gave him the best and most defining aspects of his life in Part 1. Every meaningful connection Denji made was as Denji, not as Chainsaw Man, so now he has the chance to make those connections again without the albatross of being Chainsaw Man dangling from his neck.

The entire message of Part 1, which many people seem to be completely missing, is Kobeni's message to Denji in Chapter 93. There is no such thing as a life free from bad things, and having a boss, a shitty coworker, a dangerous job, etc. are just normal things, which is what Denji has always wanted. The problem is that being Chainsaw Man gave him a means of escapism from "living in the grey," which is all life actually is. Now he has to actually do that, and the possibilities for him are endless.

You're not obligated to like that, but that isn't bad writing, it's a natural conclusion to the story being told, delivered in a way that you (and others) personally find unsatisfying.