Update: I decided to copy the configuration over from my SP2013 site, figuring that I’d at least have most of the fields and tabs I needed. After importing all three of the old configuration JSON blocks, I saved everything and set a Restore Point.
This is where it gets odd.
I closed out of the form and opened it again, and I was back on the config I’d built over the last week. Including my custom rules that don’t exist on the old site at all.
I have no idea what’s going on, but I’m very pleased that I don’t need to rebuild everything.