Another very happy user of FastMail here, with our own domain. I initially was excited by subdomain handling, but switched back to only using my main account.

Using FastMail-specific features will lock you into this specific vendor once again, one of the main reasons to switch in the first place!

To be fair, how FastMail does catch-all delivery like this is standard and easily reproduced st any mail vendor (except Office 365) that supports catch-all, which is most of them. I use a catch-all address with FastMail that is @asubdomainichose.mydomain.org and it is the same subdomain I used with my previous setup before moving to FastMail.

Using a subdomain for catch-all is great because spammers can’t easily discover and flood the subdomain.

I'm in the weird Google Apps for Your Domain limbo right now myself. I've wondered what would happen if I switched to something other than GMail but kept my google account with that email address.

I know a long time ago you could set up a Google account using a non-GMail email address but I'm not sure if that's even a thing anymore. That's what I want though. Keep the email address with my own domain that I've used for 17 years and just have a regular old Google account using that email (and keep all my Google services and purchases associated with it).

Google has been absolutely terrible to Google Apps for Your Domain users (who were often Google's biggest supporters back in the day). They've been shoved into this weird second class status where their Google accounts only partially work with Google services. I completely regret ever setting it up.

You absolutely can set up a Google account with any email address you want.

https://accounts.google.com/SignUpWithoutGmail

I use Google services heavily at work, all on a Google account that was created with my work email address. And we are not a Google shop; my employer's email is self-hosted Exchange.

You can continue using your email address for your Google account even if you've got someone else handling the mail now. You can also sign up for a Google account with an email account from any domain or provider.