You don't need to do it through the email interface. You can do it through many password managers (API key). With simplelogin and a custom domain you can set it up to use regexes and control the flow of emails to any number of inboxes. It's like a fine grained catch-all where you don't need to create the emails ahead of time and can "send as" any of them... or disable any if they start receiving spam.

If I wanted to activate a new Facebook I could just type fuckbookNOV2023@customdomain.com into the signup field and verify it seconds later. If I wanted it to go to both me and my brother, I could add his custom string (e.g. fuckbookNOV2023bruzz@customdomain.com).

I've been doing this for 2 years and only recently received my first spam. All I had to do was change the email with the service and disable the old address. Easy peasy. You can also disable the catch-all at any time if someone starts fucking your life. Then all you have to do is create a dozen emails ahead of time and assign them as needed.

Also u/umbrella

Sorry, yes you do. Because it's so interlinked in my mind I got that wrong. But deffot the easiest way to do it I've found (as long as you're using proton mail though, I guess...)

Proton doesn't offer its own aliases, they use a third party service as well. It would basically be very similar to what you already get from Relay.

Have a look at Mailbox.org if you're looking for integrated services, they offer packages with more than just email. It's a long-running German service.

Please be wary of "encrypted" mail services, they make it fairly hard to migrate away from them later, if you need to. You need special tools to get your mail out of them, and those tools are at their whim.