Crystallinity of any powder is basically dependent on the type of materials in study as well as its synthesis procedure. However, to improve the crystallinity of most sorts of the powder, annealing above the recrystallization temperature followed by a controlled coiling might induce beteer crystallinity in the powder.
If you have tried most of the annealing combinations, I would suggest you to go for flash annealing, where you have to quickly anneal the powder followed by rapid quenching. Just giving it a thought to try.
The presence of a liquid phase can promote crystal growth. Preferably you would use a solvent in which your material has just slight solubility, and of course annealing at an increased temperature followed by cooling down can help. Make sure the solvent is compatible with the heat treatment you plan (flash point, volatility) and doesn't react with your solid.