The main problem is US and SE.
If there is some pepeol that are buying Gils its because the game it self does not generate enough gils. SE fault.
A.H item are sometime sold to high or to low. Community fault.
Just like you state

The HQ ones were selling for around 10k each, they cost me under 2k to make
main quest give you around 200k and some pepeol sell item for 100k. thats 50% of the money non-crafter gatherer will make. so at some time they will end up fast out of money. so they turn to RMT for support. You end-up with the RMT gils because you support high cost of item @ AH And SE support RMT by not giving enough way to generate Gils.