The separate I/O logic board for the top / front of the tower / rack are different. ( but same in the system orientation product ( tower / rack )).
The I/O daughter board in PCI slot 8 varies with configuration? That's a new one one on me. Never heard or found that in my research.
How is slot 8 on the top or the front of a Mac Pro? There are
two I/O logic boards standard in a Mac Pro 2019. The one not in a slot is is different and is not trivially removed. Since this second logic board also includes the power button, it will be extremely difficult to get to a functioning system without this board. If missing the specific attachment cable then also stuck. The top/front button/ports are in different physical locations on the two cases which makes it more difficult to cover with the exact same boards.
Why is the stock RAM bad? I have populated all 12 slots with the stock 8GB sticks.
It isn't bad in and of itself. It is just different and slower than the other options. Mixing RAM speeds can be juggled. But the 8GB sticks across the configurations that Apple sells aren't the same.
I do have the 1TB option, but now wondering why I bought that option. I am not using that SSD at all. My main drive is an 970 Pro mounted on a Highpoint 7103.
Prudently should keep a "maintenance" macOS instance up to date on the T2 SSD. On a hard reset back to factory settings owners would need a instance there with the nominal factory security settings. It doesn't have to be used much but absolute zero is probably too far. For extremely low usage, yes, 1TB is in the overkill zone.