Can confirm what other said on the gauge question(s) , I unlocked Siegfried first and have some experience playing with her.

Use the different teams to put different element eidolons, since those are the ones visible to your friends and couple potentially get you some gems that way.

Don't upgrade R weapons since it's pointless and focus on upgrading SR weapons with attack bonuses whenever possible. That being said, I myself wouldn't shy away from upgrading those event weapons I can easily MLB(max limit break) even if they have defense bonus simply because I can enchant them to much greater heights.

On a side note, you should pay attention to weapons that drop from the disasters (normal raids) since they can drop a couple weapons each and you can easily MLB the attack bonus ones.