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.

Using your own domain name for your emails is probably the most important thing you can do, and everyone should do it. I settled on iCloud+ because I already pay $.99 a month to get more storage. For this price I get support for custom domain names and wildcard fallbacks. It's also configurable natively in the iPhone settings.

At this price it's a pretty good deal + I know I can switch provider anytime.

I highly recommend Fastmail. Self hosting is a dream but spam filter monopolies are a nightmare. Opting out of being the product is a good compromise. I pay for email.