If he decides to say 'all gold is for Thais', refer to TAFTA....

If he decides to say 'all gold is for Thais', refer to TAFTA. Same as if the junta government, or a civilian government did it.

I don't see him doing anything else directly to KCN (other than eg. the monopoly on vaccine production, could extend to something like a monopoly on gold refining - a bit of forced inefficiency, like another tax).

Plausible scenarios:
1. He helps to bring the army back into power, which was already a known scenario, with main downsides being temporary lockdowns of businesses/employees.
2. People try to reduce the power of the monarchy (reform). Leading to bloody protests between the police/army/monarchy and the people (of Bangkok in particular). This could be done with his encouragement, or without it, because the army is already pre-dispositioned that way. This would have an effect on FDI, but little on KCN directly (maybe making it harder for the expat employees/management to enter/exit the country).

I think his goals are: retain power, and live how he likes in Germany (as close to 'absolute power' as the German laws and diplomacy will allow of a foreign head of state living in Germany). So I don't think he has big ambitions in Thailand, other than partnering with anybody who will help him retain his power.