20200222 - Saturday
The i350-T4 is ordered - shipping status unclear.
I was under the impression that the PT shouldn't need drivers - that FreeBSD should just run the card as expected; I'm starting to doubt that; despite pfSense recognizing the card, it seems to hang when receiving packets on WAN.
20200223 - Sunday
I backed up the 2.5" drive to my laptop
20200224 - Monday
I backed up the 2.5" drive to the old Dell 3.5" - but it's clear that something is wrong with the latter. Once I can back up both to a different drive I'll feel more comfortable wiping the 2.5" HDD for the potential pfSense install.
I also found some good info on troubleshooting possible boot/NIC issues. I posted those on the TODOs log.
Follower @weekleyj has pointed out some helpful info, specifically that it might be worth installing 2.3.x and upgrading from there.