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.