DengueDucky

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.

It isn't a system app for my phone, so maybe. You could still use something like InviziblePro or RethinkDNS to disable its internet access without root though.

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

That's pretty cool. I might have to set it up this weekend.

What's the use case?

Would "plus" addressing work for you? https://proton.me/support/creating-aliases

I also highly recommend that guide

You can see what connections go through. No more than with GrapheneOS, and with stock I don't have to trust the mentally unstable with my security.

Remove Google apps as part of the debloat process, restrict permissions beyond what Android allows by default for all system apps, using App Ops and Shizuku. Accomplishes the same thing as GrapheneOS.