Only for paid Proton accounts, intended for individuals creating personal domains, and/or using domains to enhance personal security, and/or normal business purposes:

Normal domain name registration requires a significant amount of very personal information, including physical address, real phone number, plus more. Much of this can be publicly accessible. Many/most domain registration companies might optionally hide your personal information, but all are vulnerable to social engineering and data breaches.

I have significantly more trust in Proton.

I am willing to pay Proton to create and register domain names on my behalf, and delegate control of those domain names (via DNS record management) to my Proton account. A CERTBOT compatible API for domain management would be nice to include for LetsEncrypt SSL cert automation. Credentials for DNS management API mutually exclusive to Proton login credentials (perhaps use an API key, etc).

Proton wholely owns the domains and all required domain registration information points to Proton, but I have admin control over where DNS entries point. I can freely host my email servers anywhere (but likely still with Proton), host my website anywhere or simply use the domain name for email purposes only with no website, but Proton remains owner of the domain.

And contractually, I can pay a fee to transfer ownership from Proton at a later date, which would then make it no longer anonymous (personal info required at that stage) but a transfer would likely not be needed.

Transferring "virtual ownership" aka "delegated admin control" from one Proton account to another Proton account should remain anonymous, and hopefully only a minimal fee for the transaction.

Proton charges a typical annual fee (similar to godaddy or ionos or cloudflare or...), plus an additioinal fee if or when I want to transfer domain ownership. If Proton's related fees remain competitive, there would be no reason to ever transfer the domain's ownership.

It may be reasonable that Proton requires an active paid account in addition to annual fee per domain for maintaining domain registration and ownership.

Not sure how the general E2EE practice could be applied here. On one hand, ensuring a domain is not taken over by an unauthorized entity or reclaiming control if a hacker succeeded might require a small amount of non-anonymity to confirm the "real" virtual owner. On the other hand, opting-in for a double-blind arrangement, where Proton is unable to determine who the "virtual owner" of a domain is, could have its advantages. Personally, I just want Proton as a registrar, I do not need the double blind, but would probably use it if it were an option.