To answer directly, MAM is a great private tracker for many reasons, including their epub library. Bibliotik is supposed to be good but harder to get in.
I personally found IRC to be the best solution for ebooks. Not sure how much detail/links are allowed here but theres plenty of guides. Look for “IRCHighway ebooks” on your preferred search engine
Just want to add for anyone who might attempt this, my IRC client of choice is Pidgin. Open source and works on windows as well as Linux (not sure about iOS). Most tutorials suggest the good old mIRC, but using it after the trial period increasingly becomes a pain with its wait screen with timer.
Personally I use a Kobo Libra, as it supports sideloading books manually, supports various formats (epub, pdf, etc), has a nice backlit e-ink display, and physical buttons for page turns in addition to the touch screen.
Also doesn't support Amazon, which is always a plus. Nice bit of kit.
I use a mix. Sometimes, I'll read stuff in the foliate app on Linux from my desktop outputting onto a big screen tv.
But I also do an old kindle while lounging on couch/bed and Librera reader on phone (android/f-droid) when I'm on the shitter.
For the most part, I load different things on each and just jump back and forth between several stories. Keeps things interesting and no need to sync bookmarks and whatnot.
People don't like TPB because it is infamous for being packed with malware and sketchy uploads. I assume people were downvoting because there are a lot of safer and more reliable options for epub sourcing