So in the end, I was able to get the device online, uninstall pfBlockerNG, the reinstall it and everything seems to be working just fine. It's now running as my production device.
I will keep the other one (the one that had been my production unit before) on the shelf for two weeks just to be sure no hidden issues appear with the new one, then will reinstall and upgrade it too.
Thanks for all the help on these forums, and to Netgate for a solid product .