I really like vaemendis.net/ubooquity/

Komga is always mentioned as a good option for this. I use it for books, manga, and comics, but my requirements are quite minimal (koreader sync, opds, and mihon), so it might not work for you exactly.

I do metadata fetching using komf, documentation is a bit lacking but once setup is pretty good (I use it for both manfa and comics) and it does everything automatically. There is also a browser extension that provides a UI for it. For comics sometimes I use ComicTagger for manually reviewing tags too. I would just read carefully the folder structure Komga expects if you go with that, to avoid having weird surprises :)

BookOrbit is doing pretty well for me so far with comics. Seems to have manga options about reverse reading direction.

Even better for books too. Can do koreader opds and email books as well. (I have a kobo so I haven’t tried the kindle-orientated email, but I assume it’s ez pz)

Very good metadata ingestion. And the book/comic libraries aren’t in different silo’d worlds like they are with Calibre and its derivatives.

I’m pretty pleased so far, have found some niggles but nothing bad. And the good stuff plenty outweighs anything else. It’s all ready to go, I switched libraries quickly. And it ate up my old library with its different (not as good) naming style and realigned it automatically, which was nice.