I'm not seeing much on F-Droid, Play Store is... a mess of bullshit, as always.
KOReader! I maintain my library with Calibre and browse its OPDS server through KOReader.
Tbh, I wish the UI wasn't so wacky. I put it on my Kobo and it was really unpleasant to use. My kids have Kobos, too, and I'd love to give them something easy that can pull from Caliber.
I've found the look of the UI to be an acquired taste, and maybe easier to swallow if you're used to using open source stuff. But I'd agree that the way it works is, in places, almost unforgivably unfriendly.
But it's the "almost" that keeps me using it, because there's nothing else that works across the platforms I care about, even if the application is so, so difficult to recommend or "deploy" to users.
Moon reader is my go-to. Unfortunately it doesn't sync reading progress, but since I only read on my tablet it's not too bad.
