I transitioned from a Gmail account that is more than 10 years old that I previously used for everything. It was much less of a pain than I expected.

I set up a new inbox with Fastmail on my own domain and set the Gmail to forward everything to the new address. Then I started updating things whenever I received a forwarded message. Over couple of months everything important got gradually sorted out and I cleaned up the rest based on my password manager.

Now I only keep the Gmail around because of the Google Play purchases tied to it plus the possibility that some old colleague or acquaintance may have the old address saved. But if Google shuts it down it isn't a big loss.

Overall, the benefit of more control definitely outweighs the hassle of switching. I would recommend it.