Removing by ADB is fine if you are able to identify which packages should be removed or if they’re already covered by Universal Debloater. You should also use Invizible Pro to control what connections are made and block trackers.

It’s unclear to me how these torrents are used. If individual books are not downloaded from them, is this only to make it possible to create similar sites in the future, in case this one is taken down?

Shelter + Shizuku + App Ops

Shelter will put it in a work profile.

Shizuku and App Ops work together to give you more control over app permissions. You can restrict a lot this way.

Granted, that won’t stop the normal data collection via your IP address that occurs as a result of browsing their content.

Gboard isn’t open source, which is why I disable internet access for it.

I use Gboard but with internet access disabled for it. I haven’t found a comparable open source keyboard yet.

What’s the use case?

You can make stock Android much more private. Universal debloat tool + Invisible Pro - remove unnecessary apps, restrict internet access to only the apps you want to have internet access, add a tracker blocker list.

My experience has been that Nextcloud can do 1000 different things, and it sucks at all of them.