When you say "From my Calendar to my internet account at protonmail." I assume you are sending the calendar invitation to a third party via your protonmail account or have you contracted with proton to provide you with storage space on their servers?
There is no way of configuring Calendar or Mail such that Calendar event invitations are sent via Protonmail and everything else through Apple Mail. But you can specify Protonmail as the default account for all mail messages which includes Calendar event invitations. But you would have to be very careful to manually select Apple as the sending account for each and every unencrypted message.
There are however a large number of competitors to Calendar on the App Store such as Calendars
, and the very highly rated Fantastical
and perhaps one of those might solve your invitation/reminder via ProtonMail issue.
Sending messages, invitations, whatever via Protonmail has the added complication that in order to open or view the contents the recipient either has to have a protonmail account or you will have to send them the "pass phrase" to open the message by some unencrypted means: your Apple Mail account, text message, voice, or
So each invitation will likely require two separate communications. I would be interested to know if you find an app that will do all of that in a single transaction, although it might be possible to do it in a workflow.
I thought perhaps it might be doable in the iOS Protonmail app, but no such luck or at least not that I can find.