Well, here's the update for posterity more so than anything else.
1) I trashed PT and reinstalled it without any other plugins. It seemingly worked great.
2) I installed the HD Driver and the HDX card, first in slot 6 then in slot 5, and the Mac didn't recognize it. Oddly though, when I booted up the Mac Pro, I could hear the HDX card's fan cycle, the way it does on boot up. But when I run DigiTest, nothin.
3) Everyone had been talking about the "Belkin Aux Power Cable Kit for Mac Pro", and to use the "Cable Set 3 PCIe Aux Power Cable 6-pin to 6-pin", which I bought and used (which obviously didn't work). However, in the cable that came with my HDX card, there's a cable that has the same connectors. I tried it as well (which also didn't work), but it connected fine. Here are some photos:
Large end:
Small end:
Motherboard to first cable (the one that's the same as the Belkin):
First cable to the harness of the second cable:
Second cable connected to the HDX card:
Am I crazy? Did I miss something? Is there a reason I'd need the Belkin cable?
Back to my issue...
Again, Pro Tools is crashing on the plugins launching but it's never the same plugins that cause it to crash. It's been Trim Ha Ha! VariFi and Izotope are culprits as well. I don't suspect that it's any one plugin though, it's some bigger.
I also think that everyone who has this up and running is on OS 10.15.2, right? It might be something with OS 10.15.3. Either way, it's been a very unproductive day.
Thanks again everyone.
Ryan