I also use Fastmail with custom domain and I alias every single new account I make to know who sells my data. Through last two years I think there's been only one major incident that made me unable to see my emails and it lasted like an hour or two. Otherwise it works properly, it's fast, Calendar integrates nicely with everything else I have(like Apple and Google calendars), search is insanely quick, it's easy to save/browse through files. It's also not that expensive for what you get.
> I alias every single new account I make to know who sells my data.
I wonder if that's ever happened yet. I do the same with Firefox Relay, it's only $1/month to keep using my existing email. I have probably hundreds of accounts that haven't had their emails updated yet.
It did, I received unrelated spam onto aliases I used for major electronic chain(MediaExpert - polish MediaMarkt essentially) and two other minor services I don't remember anymore. It also happens that few of my aliases were leaked from data breaches so that also helps.
Thanks for letting me know, this is really one of my biggest concerns right now so it’s great hearing from someone who actually transferred their inbox in the same way I intend too :)