Never use your Flower Gems on the premium gacha. It's a trap. There is nothing more than the available girls before the current event in.
Use them on the priority gacha only if there is a featured girl you really want. If not, don't spend on it. Or you can try if you really need a new gold but it's not advised. But keep in mind that "increased chance of getting featured girl" means "slightly increased chance to get the featured 5*/6* ONLY if you get the 6% or 0.5% to get the 5* or 6*.
Keep your FG for the rare ticket gacha. It's the better and safer option.