I was at the same spot as you a few months ago and searched for the same.
Pretty much all of the FOSS readers sucked UX wise.
There were a few great proprietary ones on the PlayStore, but they were crammed up with ads and trackers.
Lastly I switched to Librera, which was the best FOSS reader I tried.
But even that wasn't great.
So, I'll give you a completely different advice than all the other commenters here: buy yourself an e-reader.
I did the same and couldn't be more happy!
I'm a big fan of "do one thing and do it right" (I made my phone pretty dumb and also own a digital camera for example).
Readers feel superb. They have a very comfortable display (almost like reading on real paper), don't distract you and the battery lasts for weeks.
You can also sync them easily via USB.
Just don't buy a Kindle. Aim for a more open solution, like a Tolino or Kobo.