I tried Tuta when it was still called Tutanota, but it was rather cumbersome to use. The mobile and desktop app would work reasonably well, but searching through your emails was a pain.
It also wasn't possible to use any email client on the pc. Proton also doesn't offer IMAP access, but they do have a bridge you can install for that, enabling the use of almost any mail client.
I moved away from Tuta, and while getting my mail out wasn't as straightforward as with regular mail services, it also wasn't hard.
Proton offers an IMAP bridge, which will let you use any IMAP client to download your mail and then transfer it somewhere else.