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.
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.