What bothered me about Gmail was that it was central to my life and if something were to happen and they locked my account they have zero support.
With that out of the way I feel perfectly happy with FM — no need to go further down the paranoia hole.
Right. Everyone has to keep a Gmail address as backup because of all these edge cases. For one, some domains (like yours) are just rejected. I tried to register an account with a huge corporate SMTP provider recently but they've not updated their allowed gTLD in 10 years and their devs fought me for weeks telling me it was my domain that was wrong, not their system until they finally gave in.
And secondly, many sites, like Reddit, use a Gmail address as some sort of signal of quality. You can avoid a lot of new account bans on Reddit simply by registering with a Gmail instead of your own domain.
I'm on fastmail with an xyz.dev domain and keep a gmail for legacy purposes. Unfortunately some email validators will reject my short FM address.