No ads in Thunderbird, K‐9 Mail, or any of the other mainstream open‐source IMAP clients I’m aware of.

Haven’t seen ads in Mail on iOS. It was hard to move at first, but at least I’m not harassed with prompts to download chrome anymore.

For now. Wouldn't surprise me if Google forces API only at some point.

The only thing stopping me from full proton is the clunky calendar integration.

The last time I checked there was no way to get calendar support on android / ios / macos like with gcal. There's a custom calendar app but it lacks the usability of an iCal integration

I did the inverse. I migrated to protonmail and setup email forwarding from GMail to proton. Then, every time I received an email that was forwarded, I’d go in and update that vendor or unsubscribe. After a few months, the only thing still coming from GMail was spam and I shut off the forwarder. Basically a classic strangulation strategy used by service migrations.

I did setup a private domain, but just for configuring a catch-all. Now each vendor gets their own email address. Originally did it to cut down on spam and track where it was coming from. But it turns out my personal email doesn’t get much spam. My gmail accounts get hammered with it. Seems like gmail has a unique spam problem.