I highly recommend scheduling a regular mbsync job to keep a local copy of your emails in sync with the imap server. With this it's easy to pick up and move to a different provider or recover from outages, account closure, etc. If you couple this with your own domain your email will be very portable.
I pay for a second inexpensive mailbox at Mailfence and sync the contents of the Fastmail account to a subfolder of the Mailfence account. There is rate-limiting involved, so my config is dialed down on "pipelinedepth" to ensure I don't hit that problem (you can easily overrun the Mailfence side pushing too much too fast). The cron runs via a unique user account on a cloud server every 4 hours using unique passwords…
How do you like mailfence overall?