ProtonVPN has a built-in thing similar to the "leak detection", rather it's called their killswitch feature. If you are using the GUI for Windows for ProtonVPN, go into the advanced section of the settings and enable the killswitch feature, it'll ensure that if you get disconnected from their VPN network that your computer will not send any packets whatsoever on any services. You can also set up split tunneling through the advanced section which will let you configure different services to either use Proton or use your regular internet, it's quite easy.

Also, once you pay it yearly in Proton, BTC is non-refundable even if you feel like cancelling it.

I enjoy the flexibility in Mullvad, monthly available paid in BTC - if I do not like it, I leave peacefully.