Sorry for my late reply (needed vacation)
Due to you edit I can add this information. Thunderbolt 3 is NOT only on gen 10 mainboards.
It was allready supported by gen 6/7 for PC, but never opened up the market.
Apple Mac. did open up the market with external uses of thunderbolt. With thunderbolt generation 3, USB-C and thunderbolt 3 plugs are the same and user friendly.
With the possibillity to daisy chain thunderbolt devices like external harddrives, 4k monitors and even audio interfaces or not needing to worry when you plug in a USB-C device in the thunderbolt port. (on MAC it supports both) it is a lot more user friendly than on PC. Some PC mainboards do have an USB-C port and the chipset supports thunderbolt 3, but the USB-C port does not always have thunderbolt 3 support. So it is not simply a USB-C port option. It is the same plug, but it does not work with you interface.
And yes..... I also work with Mac, but for this you don't need a Mac. (price factor PC vs. Apple is crazy)
If you want a mainboard with built in Thunderbolt 3 ports, there are only a few on the market, but they come with a large pricetag or limitations. For example The Gigabyte Z390 Designare has 2x Thunderbolt 3 ports on the back, but is expensive. Some other boards on the market are mini-ATX or ITX form factor. Most are limited in RAM slots, PCIe slots, etc.
These are NOT gen 10 mainboards, but the Designare is highend.
There are gen 8/9 mainboards that have Thunderbolt 3 support. I will keep the info related to the Intel socket 1151.
The brands at the moment that have the best thunderbolt 3 support are ASUS, some ASrock boards and Gigabyte. Most work with Add-on cards.
I work a lot with Audio Interfaces and last week I had to test an Universal Audio Apollo X6 TB3 interface for a friend. Since I do not have a Mac, I needed a Thunderbolt 3 port for PC.
The only other PC I could use (+8 in my house) My son's PC with an ASUS Z390A prime mainboard. I picked up a ASUS thunderboltEX 3 add-on card and it ran great. (Also works on the Z370 variants and gen 6/7 intel Z170/Z270 boards. Also tested it on the ASUS Z270A Prime mainboard with a gen 6, i7 CPU from my niece)
Hooked up the Apollo, tested it with the UAD software and Presonus Studio One 5.3. Runs like a train. Very stable and fast. UAD2 DSP supported plug ins work like a charm.
The only way to get thunderbolt 3 running on a mainboard without Thunderbolt 3 support (gen 8/9 and they even had succes on AMD Ryzen/Threadripper platforms) is with a Gigabyte northridge chipset addon card. There are firmware tricks and hacks with shorting pins. This way has limitations, like not be able to hot plug/unplug devices.
If you do not have thunderbolt. Well Like you stated get the USB version.