Is there an existing alternative service (read: FOSS) that would auto-generate temporary emails and manage identity for services like "Login with Apple"?
There are lots of temporary email services, most or all of which are blocked by major sites.
Using a password manager and a catch-all domain works fine for me. I don't use the federated systems (any more - I used to use Google) - I just make up an email address for anything I want to sign up for and save the credentials.