There are several linux epub readers. Though one might look at Calibre, you can call the reader portion of it without opening the whole app. Its really clumsy if you just want to read a book and whole app opens, but you can use just the reader part of it and its very configurable. Its probably the best supported epub reader out there.
I tried another, cant think of the name at moment (Lucidor?), but based on PaleMoon browser, meaning PaleMoon has to be installed for it to function, though PaleMoon doesnt have to be open. It seems ok.
If you are going Android, look at Librera epub reader. I really like that app. It runs well on Waydroid by the way and you can install RHVoice so it can read to you. I wouldnt install Anbox or Waydroid or whatever for an epub reader though. I only got into Waydroid so I could run the Kindle4Android app since there is no linux version and current Kindle4PC wont run in WINE. Kindle uses proprietary DRM crap so you have to use their software. ApprenticeAlf has no answer to their current DRM version.
https://itsfoss.com/best-ebook-readers-linux/
You might also try either Bookworm Pup or Bookworm Dog and see what apt-get downloads for FBReader. Maybe newer version, though probably not the latest. Or VoidPup, the Void repositories tend to have latest versions possible.
UPDATE: Ok I was curious. Booted into BookwormDog right now. Apt-get installed FBReader 0.12.10dfsg2-6 Its not fancy but its stable and if you search through settings you can adjust it though even with margins set to zero, it still left goodly space on both sides of text. But was able to do white text on black background and adjust font and font size. Its ok I guess. I mostly did this so could tell you the version that apt-get will install and that its stable. I didnt try BookwormPup but assume its similar. I think likely Debian only offers last opensource version of FBReader. From what I am seeing, its no longer open source.
Hey I get it, we all have our favorite ebook reader. But if you dont like this version FBReader, you probably will need to look at the other options. Or try the SNAP version. Like say though, this version is perfectly stable in BookwormDog.
UPDATE2: Yep in BookwormPup now and apt installed same 0.12.10dfsg2-6. Perfectly stable,