alci With recent updates to VPN in android ASOP, I'd give it another try without excluding anything to see if it will work out of the box.
I am too using Proton VPN but in US and for the last year (or longer), since Android 13 I think, was not able to use WiFi Calling (without any exceptions) while VPN was on - the only way WiFi Calling worked if OpenVPN TCP protocol was specifically selected in Proton VPN settings, which removed ability for the app to use VPN Acceleration.
I worked with Proton VPN tech support guys for a few weeks troubleshooting this issue and the "solution" to use WireGurard or Auto protocols, or any other, was to enable "Split Tunnelling" and exclude system apps Phone, Phone Calls, Phone Services, Dialer, and Wi-Fi Calling (I only had the first 3 of the list)- then, WiFi Calling worked under any protocol.
If IRC, there was a fix in ASOP for VPN "stuff" in October update, which is in GrapheneOS already, but I was never able to test it since WiFi Calling wasnt working in GrapheneOS. But, since the update this morning I have VPN on without any exceptions and WiFi Calling still works, which it never did in the past for more than an our on VPN (including stock Android).