I don’t understand your “problem”. If you have your domain it’s pretty painless to switch providers. You export your mail, import it to your new provider, then change DNS records and you’re good to go.

Maybe I misunderstand?

If you’re using the forwarding subdomain emails for privacy reasons yes a separate domain would make more sense. It’s pretty obvious to tie your TLD to your forwarding subdomain. Either way use whois privacy for your domain registration.

I use 2 separate domains for this reason.

I came up with a domain just for my simplelogin addresses. I see no reason not to unless you don’t want to pay for a second domain.

I haven’t really thought about this before. I have a catchall email that I make up new emails for on the spot, and if it’s something I never expect to need to email them (e.g. newsletters), I’m still using Firefox Relay addresses (@mozmail.com), it’s only $10 a year so I never stopped using it.

I don’t think that there is a need for that 🤔

I haven’t used Addy so I don’t know specific details, but I guess you could forward the emails to addresses with a prefix, e.g.

addy-site1@domain.com
addy-site2@domain.com

You can then just use sieve filters to categorise them in the folders you’d like:

Inbox
Sent
Addy
- Site1
- Site2

The only reason I’ve been thinking that you would need a separate domain, is if you are self-hosting a service like Addy: if websites realize that your domain is used for “random” addresses, your main domain might end up in a blacklist as a spam precaution (whereas with a dedicated forwarding domain, only the forwarded emails would be at risk)