Apple Book is simple and super neat. If you want you could turn off iCloud sync. I also use Bookmate because of reviews and comments from other users.
Books
Have you tried using Collections in Apple Books? I find it a really good way to organise everything.
I also like that if I open an epub or pdf in Books that it uploads it to iCloud automatically and syncs it.
I’ve tried that, but my issue is also I do not want uploads to iCloud and sync. I want my books to stay on my device without any fancy things that I can’t turn off.
You can turn iCloud off for Books in Settings > iCloud > Apps Using iCloud > Books
That way you’ll just have files on your device.
I use MapleRead. It allows you to make custom sets of books, add tags, and acts like a server to send books to other devices if you want.
Most important to me, though, is that it also gives you significantly more control of the display. The amount of words books lets you fit on a page at the densest is silly low. MapleRead lets me adjust past the optimal size and down to "actually too small" on both iPad and iPhone. I do most of my reading on a dedicated ereader because e-Ink is a huge difference compared to an LED or OLED display, but when I do have to read on a regular device it makes it a lot less painful to fit more than 10 words on a page.
It actually looks like books is less restrictive now (and yeah the margins and line spacing are adjustable in MapleRead; that was just comparing the most they'll let you fit)
This actually looks great. Gonna try the demo first but it could definitely be an option!
I forgot, but there's multiple listings, so you can use the two copies side by side if you want (though they don't share a library so it's mildly hinky). The CE version is the smaller version without PDF or server, and the SE is the full version, or there's a bundle that I think was a dollar more to get both for the split screen ability. But it's definitely worth being aware which version you're buying.