I see that there already is a suggestion to group domains.
https://protonmail.uservoice.com/forums/953584-proton-pass-authenticator/suggestions/46879126-add-equivalent-urls-setting-to-avoid-creating-main

I want to add to this suggestion that Apps should be included in this grouping process. Creating a login in an app should allow you to use that login on associated domains by default. It is reasonable that this would only be possible for well known platforms.