Not OP, but I've recently adopted that idea too. It works if you use a custom domain and set up an alias for *@<domain>. Then you can both send and receive from any addresses you come up with.

Yes, besides the solution which dalmo3 wrote about, there's a recent feature that doesn't rely on you providing a domain - https://www.fastmail.help/hc/en-us/articles/4406536368911-Ma...