I'll speak for the current meta for Katars. Since you're shuri sub and you should be using left path weapon mastery, Storm element should be your best option. I've tried a full storm skill dmg setup but what works best in my testing is the following:
Eido: 25% A/S and 45% Storm skill
Mounts: 30% dmg vs X element
Katars aren't super reliant on just one skill, so the extra storm skill on mounts doesn't help all that much when you aren't Swift striking. When "Ride the Wind" envoy node procs, it bursts pretty hard and storm skill benefits your shuri nado aswell.
You don't NEED a storm eido, but it performs better than a regular dmg eido. Against violent bosses, I just have a storm skill mount, haven't really looked into what would be best but I'm assuming a dmg dealt mount would be (do they go up to 20%? Don't remember).
I just swap between 8% dmg and 14% movespeed. If AK gave us another hotbar, I'd have ele ones. I don't bother leaving my bag open to swap things.
Also, 15% movespd tiger if u wanna zoom zoom.