On commercial motherboards, e.g. ASUS, Gigabyte, etc., the models that are Thunderbolt add on capable have a special Thunderbolt connection on the motherboard (along with the PCIe Thunderbolt card).  If the special port is not there, Thunderbolt can't be added, from what I found researching that last year when I upgraded my recording studio desktop (I ultimately went with a Gigabyte motherboard that had Thunderbolt 3 built in). I don't see anything in the Dell documentation about Thunderbolt or an add on.
(Apologies if this is the wrong board - the G5 doesn't fit any of the dropdown options & "View More" is broken) I have a G5 5090 Desktop. I've purchased an AOC I1601C portable monitor. The monitor works beautifully when connected to a laptop, but doesn't work when connected to the USB-C port on the front of the G5. As I understand it, the monitor needs a Thunderbolt port. I tried using an Asus ThunderboltEX 4, but that didn't work either. Asus support wasn't able to help me figure out why & recommended I return it, so I did. Can anyone recommend an expansion card that would work for this use case?
Adding Thunderbolt capability to a G5 5090 Desktop