Right now the Cs candidate Cs4Sr(PO3)6 is getting all its ducks in a row. As soon as crucible etc arrive there can be some hot testing. I have a lot of confidence because I have made similar before.
Out of the rest of the priority list, very few could be made below 900C. Sorry. The very high temperature furnace is significantly more effort than rewiring a garage power supply for a TIG welder. The issue of flux contamination remains observable in, for example, monazite structured orthophosphates. Pb flux, Pb contamination, Mo flux, Mo contamination. But monazites can be made at 150C and you get single crystals.
I had a similar discussion locally some years ago, after I made some nice NiP2O6 at 400C. The feedback then was "it ain't a silicate, don't bother". My sense is that silicates are best prepared by hydrothermal process, because those have been shown to be efficient and clean. Sorry that I cannot offer anything more positive.