Well, it appears to be that the pfSense GUI is simply hiding the IPv4 Remote Networks option that I need to set on the main page, since I've selected a "Remote Access" mode. This is somewhat disappointing since that isn't even an OpenVPN "thing", just a pfSense "thing" to determine what GUI options to show and what options to hide.
Guess I'm off to file a bug report / feature request to ask that we get a 5th "type" option for OpenVPN servers called "I know what I'm doing, don't hide any options from me."
In the meantime for anyone running into this, I think the only way to address it without having to create a separate server on another port so you can get run it as "peer to peer" is to assign an interface to the OpenVPN instance which you can then use to assign a gateway and a static route in the main pfSense routing configuration.
Ugh.