Exterior Lights, Looking for a Pro's Perspective/Knowledge

From what I understand, outside of the US, transition altitude varies greatly? Is it regional, based on the ATC, or even more varied than that?

Yes, correct. TAs can be as low as 2500 feet! Hence the rule “below FL100/10,000 feet or below transition altitude/level”. This caters for countries with TAs/TRLs below 10,000 feet and also for TAs/TRLs above 10,000 feet.

Thanks Andreas