Since nobody answered, I will by myself.
I finally bought the Teclast X5 Pro aaaand: yeah, in fact there are several mentions on Thunderbolt over USB-C on Bios. It doesn’t come activated by default, you have to do it on your own. Bios is accessed by spamming “Esc” on start.
There are also several mentions on external GPU usage over Thunderbolt, which are deactivated by default. You have to activate it, and when you do, the default is “AMD only” and you may change to “AMD and Geforce”.
I haven’t had the opportunity to test a Thunderbolt device, nor a External GPU over Thunderbolt. I’ve only heard it has been done on similar chinese tablets.
I’m updating the BIOS with the file provided by Tech Tablets tonight, and as soon as I have new tests done on Thunderbolt I’ll come here to share what I’ve got.