I personally find Fastmail's spam handling a lot worse than GMail's: and I get a lot more spam in my GMail address, due to it being a 22 year old address, which I still use for initial sign-ups, before then changing account emails to my fastmail one after a few months.

Very likely a lot of the difference is the types of email each address is getting.

First time I hear about signing up then change email trick. Does it help reducing spam?

I'd like to think it does, but honestly I have no idea as I have no way of comparing to not doing this.

I am not a Fastmail user but I completely doubt this. I have been using Gmail for 20 years and I have never seen a spam mail land in my inbox which has always impressed me. Everything else I have used from time to time does miss a few, which I guess it might be acceptable if you want to move to another host.

There's like 1 spam message in my Gmail inbox every day. Some of it is so obvious that an LLM running on a Raspberry Pi can correctly classify it. For example car insurance ads.

(My email address is on my home page. It's also my username on over 100 sites that I signed up for over 20 odd years.)

Are you kidding me about the spam? I have 3-4 spam emails land in my inbox every day on Fastmail. It's horrendous.

Doesn't Fastmail provide aliases?

Infinite email masks. Doesn't help much once your real email address is out there though.

Create a rule that moves messages with real email address to spam where they are deleted as soon as needed, then create tmp2026 alias to serve as your new default email address.

I have more than that on gmail and none in Fastmail. My gmail account is way more over the place than my FMs though.