This is a blocker to moving from Google. I've got an age old gsuite workspace plan (free) and catch-all is true catch-all which is really helpful. Whilst I want to move to proton I feel like it's still 12-18 months away from being a suitblae migration target.

True catch-call (no account setup), more than 3 domains and better / custom pricing on what you want.

+Alias is not enough. I get limits on Proton owned domains, that makes sense, but limiting the number of aliases used on my own domains should not be a thing.

I get that there may be technical difficulties with this, regarding encryption. I think the obvious solution is to generate a "catch-all" key that can be used for these addresses that haven't been given an "address" slot (because they aren't used much).

You could use Plus Addressing or +Alias as Proton calls it:
https://proton.me/support/creating-aliases#aliases

There is no limit in the usage of that.

Agree. It's convenient that I can use anything before @mydomain.com, but then sometimes the vendor expects the return email to come from that same email instead of my default. It is a hassle to have to create the accounts one by one. I don't know the technical requirements or limitations that might make this difficult, but I do think that anything @mydomain.com should be considered ONE account.

Isn't what we really want unlimited "forwarder" addresses (distribution lists, groups, whatever you want to call them) that relay to real (paying) users ?

Proton recently acquired a company called SimpleLogin that provides this very functionality, among others. The only caveat is that you'd need to setup the email on a different domain or subdomain to the one currently using Proton's MX - e.g. <whatever-name-here>@catchall.address.com

You might want to check them out: https://simplelogin.io/blog/simplelogin-join-proton/