This would be great

To clarify; a user with account username abc could have the proton social account Goku@social.proton.me

A "Proton Social" app/instance, something that might have come to mind once or twice.
So what should the instance be? Mastodon, as it's currently the biggest and most compatible with other Fediverse content.

A key reference for this due to a similar but not same setup is Vivaldi Social's instance of Vivaldi, using Vivaldi accounts. In a similar manner, Proton Social will use Proton Accounts, but the account username is not tied to the proton account. (during sign-up, username is selected then to complete registration login to a valid proton account similar to simplelogin must be done)

However, as servers are not free, same for moderation, this instance will have requirements for joining: a proton account that has had 6 months of proton unlimited or equivalent payment, scaled for the region of the user. This does not mean you need to be currently subscribed or even ever had proton unlimited; for example subscribing to merely calendar plus for long enough will make an account eligible for this.
This protects Proton Social from spam accounts and similar.
Accounts for Proton Social are of course not automatically created.
Log-in is solely via logging into Proton Account though.

During sign-up a warning that content posted may be retained forever by other Fediverse servers must be displayed.

Server rules:
1. Follow Proton Ag tos (https://proton.me/legal/privacy and https://proton.me/legal/terms)
2. Advertisements (paid promotions) must have #advertisement tagging which users by default will have blocked. Paid promotions must explicitly state such, accounts owned by businesses and corporations must state so in account description.
3.Both local and Swiss law must be followed.
4. Explicit and/or Violent content MUST have content warnings.
5. Accounts may be deleted by Proton Ag for any purpose/reason at any time without prior warning, and under Proton's inactive account policy.
6. No discrimination of any form (incl. racism, sexism, homophobia, transphobia, xenophobia, casteism, ableism, etc.)
7. No incitement of violence or promotion of violent ideologies
8. No harassment, dogpiling or doxxing of other users
9. Do not share intentionally false or misleading information.
10. Proton Ag and subsidiaries retain the right to moderation in full of Proton Social; attempts to bypass or promote bypass of instance rules are considered violation of the above. While users may choose to self-moderate, users may not attempt to moderate others without explicit consent; repeated attempts of unwilling moderation are considered harassment.

Note that while a similar suggestion (https://protonmail.uservoice.com/forums/945460-general-ideas/suggestions/47630906-proton-social) exists, this suggestion has requirements to join (see above), and intends to be overall more specific.