Should have been more patient/persistent, and kept working on it before I posted here. Eventually sorted this out myself.
For anyone referencing this article later, here's what the issue was:
I messed around with DNS settings just after getting the VPN online, because I want all internal DNS resolution to go to the server-side PFsense box (it's acting as DNS resolver). I had put an entry in the general setup, specifying my server-side pfsense box as a DNS server, with my client-side ISP IP as the gateway. This was causing a static route to be entered into the table, and was the root of the issues.
I still have some things to figure out with DNS, but the original issue I was posting about is now resolved.