Just do it! You'll have to choose anyway if you are one of the many folk here who use(d) a grandfathered free Google workspace plan with your own domain: it ends on June 1st, or thereabouts, and you'll be shunted into a paid plan (although there seems to be a waiting list for a free plan).

I did it last week. I signed up for Fastmail, followed their excellent documentation, and now only have a mandatory (new) Google account for a few apps in the Play Store that are not available anywhere else (but nothing paid). If I lose access to my Google account, I lose nothing.

My Fastmail migration basically went like this:

* Clean up mailbox, truncate mailing list folders.

* Copy mail to Fastmail using their importer.

* Change domain settings at your domain host (changing MX-records and a bunch of others); mail now goes to Fastmail.

* Set up mail and calendars in Thunderbird on Ubuntu and K-9 Mail on GrapheneOS.