I just tested ExpressVPN with openvpn using GitHub - tasket/Qubes-vpn-support: VPN configuration in Qubes OS
It works well on a debian-10-minimal on QubesOS 4.1
The hardest part was to find the openvpn file on the ExpressVPN website. Click on setup in the upper right corner and then chose Manual Configuration. You will be able to see the username password you will be using and to download the OpenVPN configuration file.
Hope it helps!
Bishop