The Gigabyte GC-Titan Ridge kinda works on the Designare Z390 but not as well as the onboard TB.
What make you think your onboard Thunderbolt chip is "hosed up"' ?
The chip to flash is just a very basic 1MB flash ROM, it's replaceable (with some tricky SMD soldering-fu). But it's very hard to brick unless you physically damage it. (But it's also possible you damaged something else on the board if not careful.)
If it doesn't boot, it may just have a corrupted firmware.
If you can't read it anymore you may just have a bad connection to it.
If you manage to have a good and stable connection to the chip, you may be able to rewrite a correct firmware and solve your issue.
What method did you use to flash the chip ?
The most reliable way is to disconnect everything on the board, take it out on the flat surface :
- Use a 3.3V CH341A programmer or a Raspberry PI with a SOIC8 clip
- Stabilize the Vcc rail with additional capacitance, some recommend one 47µF capacitor for the Designare, I had better results with three 10µF.
- If it doesn't work, add two pull-up resistors (1kΩ-2kΩ) connected to /WP (to force disable write protection) and /HOLD (to force the chip in a resume state).
Here is a picture of my setup :