Let’s not forget that the point of Proton’s services is encryption. The more points you want it to go through (Proton to Cal{SVC} to Android calendar app to watch), the more you have to relinquish that security if they aren’t cooperative/companion apps.

For example, I use Proton on the web (through Tor, actually) because I get the security. If I used the Bridge, I’d be decrypting a layer just so I could use a favored email application.

Yes, you can, and I would also vouch for Nextcloud.

Asus RT-AC65

My router’s admin console, OOTB, gives me the option to either deny individual devices (based on MAC, etc.) access to the external internet, or create a second (or third, etc.) WiFi network that, itself, is not connected to the outside.

Perhaps you have similar settings?