I confirm that ProtonVPN is easy to install, because a GUI can be download by Solus directly.
And the protonvpn-cli is available too in Solus.
To install protonvpn-GUI and explainations: https://protonvpn.com/support/linux-vpn-setup/
ProtonVPN seems to worf fine, I'm going to test it a few days in the free version, I think I will pay for ProtonVPN-plus, it's a little more expensive than NordVPN, but protonvpn seems easier to install.