Another option is to go the other route and choose an alternative to your port forwarding, like Cloudflare Zero Trust, Tailscale, and maybe ZeroTier (not familiar with this one) - all can be used for free.

Despite being a mullvad user for some time, I’ve never used their port forwarding service, so I don’t know if it could fit your needs like mullvad did. But Cf tunnel lets me access specific ports in my homelab network from the internet, after authenticating, no matter what’s in between: so both my remote machine and home workstation can have their VPNs enabled and no knowledge about each other’s IPs.