Unfortunately it's still not possible to do without a PPTP server. It's simply not feasible to use OpenVPN, Wireguard, Tinc, etc without having to install additional software on OS's, devices, etc.
So I have resorted to setting up a FreeBSD VM that's running mpd5.
What do I need to do on the OPNSense Firewall to allow traffic to the PPTP server? I already did the usual NAT port forwarding for TCP port 1723 and the GRE protocol. While I can "connect" it's failing on LCP paramater negotiation "LCP: parameter negotiation failed". I suspect the auto-created firewall rule is wrong?
Set the associated firewall rule to "Pass" in your port forwarding.