I have installed an aftermarket thermostat for now. Was able to source some M6 * 30 mild steel studs to replace the existing studs. All installed, new coolant added, burped to ensure no air pockets and road tested under a wide range of driving conditions (freeway, stop start traffic jams, suburban driving). So far so good. Temperature rock solid, indicating just under half way on the temperature gauge whereas before it hardly moved off Cold. Thanks for all of your help.
I do have OE studs on order from a Toyota dealer so might end up putting those in if they ever arrive.