which elements the best ones to pick up then?
Ice and Thunder, if only because that's typically what other nukers use, so you'll have things like Ice Threnody or Lightning Threnody on. They also MB Light and Dark.
I'm swapping Thunder for Wind because I like Garuda and I don't have Threnodies that often.
All of them do the same damage. Titan can Stun, no others do anything extra.
The commom approach is to get Titan and whatever else you think is nice.
It's nice in theory, but I don't think I've ever ran into a situation where Chaotic Strikes wouldn't work.
I don't know if anyone's proven that the Stun from Geocrush is earth based, or if the damage is earth-based and the stun is still thunder-based. I've heard speculation both ways, but never bothered to try.