Well, I'd normally suggest moon+, since I've used it for a long time on dozens of devices with no issues that weren't in the source files themselves (which I would normally suggest calibre to fix those, but you mentioned not being fond of it as well).

The next best app I've found in terms of rendering the files correctly is librera. After that, I'd go with maybe fbreader, or koreader for non android devices

Epub is definitely not write-only. It's pretty much the default format for ereaders outside the Amazon ecosystem. That doesn't mean it's perfect though I've written something like five books at this point (depending on the length one thinks it acceptable to call a "book"). And, even with paying attention to my formatting during my finalization process, I get errata from the conversion process

Calibre is the most consistent at converting the base document files into epub, but it still manages to either misread my formatting, or not be very clear in what formatting is supposed to be used to achieve a given result once converted. Most of the word processing software (I use libreoffice, but used to use others) doesn't do any better.

So what you may be running into is that the files themselves have some little anomaly from their creation that's rendering weird in different apps. I can't say for sure that's the case, but I have seen exactly that problem before with my own work.

Thing is, other ebook formats aren't better. Mobi is actually worse to deal with, and tends to cause its own problems. PDF is consistent if you format the origin file correctly, but isn't great for books without extra work. The the other options are significant downgrades from there, imo as both reader and writer.

All of that said, there are a few other apps that I've tried that weren't bad, but didn't match my preferences. Readera is solid. Prestigio usually rendered fine. Pocketbook was kinda bad, imo, but only in terms of ui, not rendering.

On fdroid, anx reader is good. Lx reader is clunky, but rendered mostly okay. Those are the only ones from fdroid that aren't in the play store that I've used to test my own books.

Overall, librera is probably where I'd point you since you weren't happy with moon. The ui isn't great, but it works okay. The rendering is at least consistent, so you'd know that any issues would be with the files rather than the reader.

Edit: looking at one of your other comments, I noticed that images and tables are where you ran into trouble. That comes down to the files themselves usually. I think you may want to crack open calibre and convert what you have into a few formats and see if any of them preserve the formatting better in a reader app.