well, it's an ending. A bit "rebuild from the ashes" maybe, but it's fine. I don't like how so many themes got dropped by the end and remain unfinished, from the gender stuff to Internet culture to the humanity of zombies to the nature of survivors. Heck, the manga doesn't even bother explaining what was going on with Hiromi and why the ZQN are fused into giant monstrosities. Not even sure what making that a plot point in the first place did for the manga honestly, besides turning it into Shounen Battle Manga #56709. Same with the alien theory stuff, but at least that helps explain why the ZQN leave.
I really don't like how the Kurusu storyline developed. It started off as a way to explore the superiority complex that a ton of Internet nerds have. Then the plurality/multiplicity thing got introduced, and there was a second cult dedicated to some celebrity, and the other monsters started happening too... it kind of just never developed into what it needed to be. Not to say that it lost sight of the delusional superiority theme (the Naked Emperor is still ranting about that at the end), but it never managed to capitalize on the theme and develop it into an actual interesting idea.
And in the end, a bunch of semi-important side characters never get a resolution. Hiromi at least has her moments of exploration and gets to figure out her feelings towards Hideo once she becomes swarm queen (even if it does come with a bit of Mindbreak the Cutie), and her conclusion is to let him live and be unhappy. Oda-san on the other hand is just purposefully unresolved as a character, and her oft-mistreated sibling likewise never gets explored (oh I know they get scenes, but it's just a kind of gawky "here's what happened isn't it weird"). And of course, Korori/Nakata and Hideo never meet and resolve the whole mess of emotions there. But at least that has a poetic sadness to it, a bit of "he never saw Molly again", you know.
One last thing: I appreciate the audiovisual hallucinations coming back. I know that thematically it means he's unable to face the reality of his loneliness or whatever, but it's realistic. Real people dealing with hallucinations don't usually just "get over them" at some point. It's a lifelong thing. He's not worse for it, just different.