I switched from Tuta to Fastmail. Tuta was mostly fine, but being restricted to only their client was really annoying. I can use Fastmail with Thunderbird/FairEmail/etc just fine.
I get what Tuta is trying to do with encrypted email, but IMO they're better off joining up with Fastmail on better specs that allow for the things they want to do, instead of limiting what email clients you can use.
The inability to use my own client was the deciding factor of why I did not go with Tuta when I was email shopping. They've got good features and a fair price but that one decision is a serious turn-off imo.