Realistically, if you aren't in a "serious" crew for Unite and Fight, you don't really need to use Sunstones. The only stuff you would need to sunstone are Primal summons- which without Gacha weapons are worse than f2p M2 grids, and if you have a Primal grid with gacha weapons in it, you probably are committed enough to the game to be in a crew. The only other stuff that you would want to sunstone are Bonito, Lucifer, Bahamut, and the 120 series (Grimnir/Shiva/etc), and for the most part, those are going to be easier to find on your support list than a M2 summon.
You also should not be using Sunstones until you have enough to fully uncap the summon, as you might end up drawing something better after you have already used 1 or 2 on a summon.