So I finally rolled something decent and got a Brynhildr.
I am really unsure how to proceed now. Do I keep her or wait for a decent dark/ light SR.

If this game is like Granblue (which seems likely now), going dark or light wouldnt really put me into a disadvantage regarding the weapon pool. As I only would have to grind for one pool, instead of two if I were to go fire ( Assuming the drops chances for light/ dark weapons are 1/2 of the other elements like in Granblue (wind not included!)).
But the advantage of going dark/ light would be, I only have to collect one color of characters, which counts for a lot in a gacha game ...

Any advice here ?