My honest advice: If Proton fulfils your needs, stick to it. There are currently no concerns that outweighs the benefits Proton can provide.
A possible alternative would be Mailbox(.org). I would recommend against Tuta, due the absence of JMAP/IMAP (vendor-lockin) and lack of PGP encryption support (means you can only send End-to-End encrypted emails to other Tuta users).









There are various entities who can take your domain away, people seem to forget that.