Exactly the same here. It's not perfect... it's slow, it doesn't work with your mobile mail client... but it's better than the supervillain levels of evil that Google is pushing out.
Ah I guess I didn't think of that use case, it being the Proton app. I think I read somewhere it's possible with the Windows client, through their Bridge software. I haven't had the need to use the Proton client for other email accounts personally.