First off, are you low on half elixirs (I'd say less than 50)? Get more of them first.
Next, are you a free player or want some more Kamihime? Get the gacha tickets as they're cheap.
And honestly, the SR weapons are cheap too, but not worth it until they start getting 2 skills on them. You can farm SR Assault weapons elsewhere and Defender should come from SSR weapons that also increase assault or pride (or vigor later down the line cause it's also a good damage increase when used right). When you hit my point where your grids are full of SSR weapons, SR becomes useless (I have 2 SR weapons left among all 6 elements since I'm a Day 1 player)
Top priority is the SSR items... but the Koihime crossover Eidolon isn't too great so she really should be sold for eidolon orbs as even the SR disasters do more than her. If you want to keep an Eidolon, do so. If you don't, get one copy to lv40 for the extra magic jewels then sell every copy of her you get. EIDOLON ORBS ARE VERY HARD TO GET.
As for the SSR weapon, always aim for weapons with Assault or pride on them as a start. For ones that don't have either, get them anyway cause they make great skill leveling fodder. Either you will use the weapon and raise it's power, or use the SSR weapon for easy skill leveling as it currently guarantees 100% chance if it's skill level 2 when used as a material. Even skill lv29 weapons got 100% success like that.
And always do the highest difficulty so you can get as much as possible. When you start clearing expert and eventually ultimate, then it's not too hard to net maybe some magic jewels after everything else. Don't bother with anything else until you start getting magic jewels as the SSR, gacha tickets, and half elixirs have far more value and priority.