Using @fastmail.com makes no sense to me. If being locked into somebody else's domain is acceptable, then Gmail is a better deal.
And if you still insist on using somebody else's domain, then all bets are off after you stop using that account. First of all because the service provider can always change their policies. Just like how when Yahoo announced in 2013 out of the blue that they'll start recycling usernames.
Fastmail is a paid service, you own that username for as long as you keep paying. The notion that you can take up resources, for free, for all eternity, is only sustainable for big companies like Google. And given how crowded @gmail.com is, I wouldn't put it past them to start recycling in a couple of years.
If someone takes over a domain they can also get https certificates, reset all their passwords and do all kinds of other things that are related to it if there's no second factor. That's just what you have to accept if you don't renew a domain.
If paying for it is a problem, then pay it in advance for 10 years and setup automatic renewal. Also leave a digital will. You probably want to do that anyway.
> they aren’t reading your mail to target ads at you
Neither does Google ;-)
This is true since 2017 : https://variety.com/2017/digital/news/google-gmail-ads-email...
They recycle your email address once you stop paying. A great black hat technique is to gather up all Fastmail addresses you can find in the wild and poll them to see when they're available again. Then launch social engineering attacks from them to impersonate the previous user. I found this out the hard way.
Amateur hour. At least DHH has said hey.com locks your email address for eternity, the only right way to do it.