What are you looking to get from TB3? The advantage of a desktop is standard PCIe slots, and with USB-C (3.2) external speeds are still nice.
I'm looking at doing a video workstation build - stitching video (Insta360 Pro2) sucks with my laptop.
Yes, but I want my storage separated from my computer, partially for easier upgrades down the road and partially for size of storage. With enough drives I should get at least as much speed as from my SATA drives (even as they are SSD only) but I don't think USB will cut it. I know I can add drives in the desktop and I did, but I feel it would be easier to separate the upgrades. Another reason is that I want my computer to be silent; if I want a lot of cheaper storage I would need to go back to regular HDD, thinking of 6 to 8. Putting them in a box that I can move farther away from me would decrease the noise.
I would also need to upgrade my NAS in the next year or so, therefore I contemplated using a NAS with dual 10GB, or with the newer 40GB or 100GB but it looks a little too complicated for my level, particularly if I need to troubleshoot it. I could try a NAS with both TB3 and 10GB for best of both worlds but I would like to see some real world testing.
Some of the X570 mbs have thunderbolt don't they?
I didn't check but I know on the native Intel side there is a paucity of motherboards that can reliably provide TB3. So far the Gigabyte Z390 Designare is the best candidate that offers built-in TB3, without additional cards. On the built-in side is a Dell XPS Special edition with the add-in card, or the latest iMAC 27 if you want Mac.
Searching now it seems that only ASRock has a few that will need an add-in card.