I would think that ideally you would want E2E encryption that was on by default for everything. Then rather than turn it off, you would also want the ability to

I would think that ideally you would want E2E encryption that was on by default for everything. Then rather than turn it off, you would also want the ability to configure a server to effectively MITM everything (and the client to cooperate in this, and notify the user about it). Such an arrangement is more or less what happens with key escrow, a feature which I believe at least one self hosted cloud stack offers (sorry I can't remember its name off the top of my head).