Buying your own domain and then just pointing your mail records at a new host is the best way to handle email migrations. Once you get a new host with MX record support, which should be most of them, you just go update the record and all your mail is done being moved immediately.

You will have to hook things to yet neither address, but only one more time.

A year ago I left a 2 year proton subscription when I was only 6 months in. I avoid subscriptions now.