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.
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.
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.
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)