I've been using this a few months and recommend it as well. It's serviceable but feels a bit janky. Scrolling and switching between view styles tends to leave black boxes on my device, and the UI doesn't always disappear while scrolling. But I'd still recommend it for how light and fast it is on top of being FOSS
I've been using MuPDF for pdf's and Libera for epubs, both on f-droid. Libera can also read pdf's but for reasons I don't remember, it wasn't as usable for them as MuPDF is, at least for me. So I stayed with MuPDF.
What sort of network library integrations are you referring to? The version I install directly from repo has Project Gutenberg and Internet Archive preconfigured, and I can add my own Calibre and OPDS libraries too.
Edit: the Play Store version (Pro) is also available via the repo, along with the F-droid release. Another reason I avoid F-droid and install direct from repo using Obtainium.
KOReader. The app is multiplatform (also for some e-readers and Linux) so the UI is not among the most beautiful (but I actually prefer it over Librera), but it's feature packed, and does really well what it's made for.
I had Readera Premium purchased 3 years ago when I wasn't aware of really great free softwares' existence. I had bought it because of it's simple interface and thought it could help me with OPDS (auto sync between devices or other apps), but it didn't help me other than by just being simple. That's why I am currently hosting my own OPDS server (Kavita) to sync my progress and read from it's web interface and Librera app (really powerful and open source). I am thinking of moving to Calibre Web from Kavita because of ease of uploading books. You can try Librera which is a really great and powerful pdf/book viewer/reader with/without OPDS.