I liked Kotatsu for some features, but I ended up trying Mihon after seeing it mentioned a lot, and it has most of what Kotatsu does.
And I also had trouble with Kotatsu's sync. It synced my library, but it wouldn't update read chapters between devices. So if I read up to, say, Chapter 14 of a manga, then did the sync and read Chapter 15 on the other device, the first one would still have me at Chapter 14. Also, not really sure why, but activating the sync on the second device was kind of a pain. I had no trouble setting it up on the main device, but then the second one wouldn't open that screen where you can load your library. I ended up having to make my way through some well hidden settings outside the app (as I figured out that the screen it was meant to open was basically a redirect to the phone's settings), and it wasn't even worth it, since it's not properly being synced anyway.
Yeah, that's fair. You said you were curious what other recommendations there are, which is why i mentioned Mihon.
I'm not sure if Aniyomi is going to be affected by what happened to Tachiyomi, cause I haven't used Aniyomi in years, but hey, if it still works and you like it, keep going with it.