"Friendly fire is on"
"Ok" Walks through fire
"I THOUGHT YOU SAID IT WAS FRIENDLY!"

Wait wait wait wait, you can connect Tachiyomi to your own manga host? Well... shit. I know what I'm going to be doing next.

I highly recommend AudioBookShelf . It's mostly geared towards audiobooks/podcasts, but it has a decent reader built into it. That being said, it's not without it's flaws, like some document file types aren't recognized, and not properly syncing progress occasionally.

Edit: I should mention it's actively being developed, devs are active on discord & GitHub, and it's getting better all the time.

I used to run CyanogenMod/LineageOS on my older phones, but since my S8, I've been using stock ROMs. Honestly not even sure a custom ROM exists for the S21. I'd love to make the switch if it does, however I know Google Wallet (Google Pay at the time) had a huge issue with custom firmware/ROMs and I don't imagine it's gotten any better over the years.