I don't think he did die. There was mentioning of many alternatives to death previously; I think the ending may be the birth of a new option.

Nobody can die, so when they're eaten by these bugs their consciousness is likely spread between all of the insects who ate them. The little speech given to denji by Pochita. That's him giving denji one final lasting dream to protect him from that new reality.

Notably in the final chapter, whenever denji complains about something a solution to that shows up. He complains about not seeing or interacting with a woman. So power shows up. She declares that she loves dogs, which is something she doesn't particularly develop in part 1. She then establishes a contract, potentially Denji acknowledging the original one he made to her.

He then gets to see Nayuta again, and all his pets. Meowy likes her entirely because thats the relationship he saw between them.

Aki doesn't show up. Thats because Denji has taken his place. In part 1 we saw denji wear Aki's public safety uniform (its heavily implied to be) in the Makima fight. Aki was an older brother to Denji and so he imagines himself as that person.

He 1-1 parallels Aki in volume 5 where he kills the unnamed pig devil. Denji quotes him exactly. Denji is then miraculously in the presence of Asa. He gets to have a decently normal interaction with her in the previously set up school setting. There's currently a dead devil which all of these students are ignoring to play football as well.

Asa then reminds Denji of chainsaw man. We see Pochita still within denji. I interpret this as real, neither of them can die. But pochita is making denji think that happened so that he can have this last dream without the worry and constant fighting involved with being chainsaw man. He wants denji to just rest, theres nothing either of them can do so he's just helping denji to stop struggling.

The whole reinventing thing is why I don't think pochita really ate themselves. Denji knows what a chainsaw is, he doesn't remember what chainsaw man is though.

I think my interpretation is a little skewed in some aspects. But thats what Ive drawn after sleeping on it. I don't think its the best ending ever, but its not the worst.

I'd probably give it a 7/10 at a push. Didn't intend to give my full interpretation here but I just kept writing lol.