Currently I’m using booky mcbookface but the app is really lacking and the scrolling is not smooth.
Edit: I came across Lithium and Readera which both don’t seem to collect much information compared to other apps. Not sure if they’re open source though.
I’ve had the large file issue with Librera too. Bundled epub collections with absurdly large page counts have sometimes been extremely slow to load. I’ve had better luck recently, so it might be a partially solved problem.
Good to hear. For context, I made the switch late last year, so my experience may be outdated.