With Proton that is easier, but if I'm correct you have to use their app or webmail and if you want to use Outlook for example, you have to install Proton Mail bridge. So still extra's to configure while Gmail and other mailservices can be configured automatically.
Next to that, this is just like Gmail a special seperate mail service. Nice for customers who want to mail more safe/secure, but it's not DA's competence. DA is not a seperate mail service and this is not a simple customisation either.
DA is just presenting a GUI shell to configure other applications. So for these kind of changes, you have to be with those applications like Roundcube and Exim.
But I understand that some would like it.