You're obviously correct for ITX, but I was surprised when I found out that (on larger boards) you can simply get a Thunderbolt PCIe card from Asus, Dell, etc. and it will just work. It appears that in many cases, the infamous THB_C header on mainboards doesn't serve any purpose.
Having used only a MacBook for 4 USB-c/thunderbolt ports for several years, and having moved all of my stuff (even my cordless shaver!) to USB-c / thunderbolt, I was really surprised and annoyed when I built an AMD gaming PC and realized I could only get 1 or 2 USB-c ports and almost no options for thunderbolt. It made me realize why most mice, etc are USB-a still.
Well up until late last year or so, Thunderbolt was an Intel only thing. They then released it to the USB consortium and USB4 is an extension of Thunderbolt. So this motherboard the OP used is probably one of the first ones with USB4 compat.
I expect a lot more AMD mobos to support thunderbolt in the next year. Music producers especially really like the interface for it's low latency and bandwidth, and Ryzen would be an awesome pairing for the resource hungry applications they tend to gravitate towards.
Anandtech article announcing Intel's Maple Ridge - a chipset to enable Thunderbolt 4 on future motherboards regardless of the CPU (i.e. Tiger Lake not required.)
I do something similar to the post's two-cable configuration:
- Mac to Caldigit TB dock to two displays. - PC to one of the displays - Keyboard and mouse to a USB switch, which connects to the PC and the TB dock
It's kind of a pain to manually flip both the USB switch and the monitor's input.
This app can theoretically help by programmatically toggling the display's input when particular USB devices are dis/connected:
https://github.com/haimgel/display-switch
.. but compatibility is spotty, relying on a rarely-used hardware feature in the display that seems to not be reliably implemented ("DDC/CI").