First I would check D3 and magnesium levels, if they haven't already been addressed.
Stick with low-dose (1.5-3mg) of sustained-release melatonin if you're going to try it.
If you try GABA, I'd go with phenibut aka PhGABA or PharmaGABA. Know that a tolerance is often quickly reached with this. If this works, then I would switch to a supplement that affects GABA pathways (ie. passionflower affects GABA pathways).
An Adrenal Stress Index saliva cortisol test is always one of the first tests to do in these situations.