Love fastmail. 500 aliases means a unique email login for every single online account. Hope they fix this soon.
You can also send with a wildcard - the big desktop email clients, as well as the FastMail web interface, as well as FairEmail on Android all let you type something for the part before the @, so you can send from anything@yourdomain.com.
I have different categories per services: paid services go to @paid.mydomain.tld, social logins go to @social.mydomain.tld etc. So that means something like paypal@paid.mydomain.tld or twitter@social.mydomain.tld.
Edit: using Route53 for DNS subdomain wildcard.
Any specific reason to do this over the usual way (paypal@mydomain.tld, twitter@mydomain.tld)? How are you benefitting from the extra hierarchy?