True, in deed I was told by Proton staff member that ONLY IF you first create the SimpleLogin account, then link to Proton account, all the SimpleLogin domains can be synced to Proton Pass. This is irreversible and must be in done in such way.

So, like you and me, we created the Proton Pass account then link to SimpleLogin using Login with Proton account, we cannot use SimpleLogin domains in Proton Pass or use Proton Pass domains in SimpleLogin. This is a known limitation and sees like the team is not planning to cater this.

What we demand is SIMPLE - but not catered.
Simply allow paid users to use all available domains under both Proton Pass and SimpleLogin, regardless of the way they created their account. e.g., use .passmail.com in SimpleLogin and use .simplelogin.com in Proton Pass. This is a technically feasible and currently developed feature, but not available to customers... Please, Proton, consider taking action to make it available to us.