What's Del Rasha's play style? Make a bunch of Slow Times, then shoot things inside? Does it use Serpent Sparker by any chance?
A lot of slow time for crowd control, teleport around and see meteor wrecking stuff, all you need for it to work are 8 set pieces and a RoRG, the rest are refinements, it doesn't need any specific weapon to work although an Aether Walker is essential for its full power.
For additional pieces you want:
Crown of the Primus for additional Slow Time power
Strongarm Bracers/Ancient Parthan Defenders for additional damage/survival
Nilfur's Boast for meteor damage
Focus+Restrait for obvious reasons but Convention of Elements+Unity/Halo of Alyse is a strong candidate.
For weapons you have tons of choices, Aether Walker, Serpent Sparkler, Thunderfury, whatever you got is good, also a cubed The Furnance is the best to have.
But none of the additional pieces are required for Delrasha so you can build it up very quickly and work from there without having to play on lower difficulties because you don't have the power. I started to work nicely in T7 and GR35 before I have any 6pc set because 4pc DMO and 4pc Tal alone are already powerful enough to wreck T6.
To add to this, high GR playstyles are to move as fast as possible and mostly focus on killing trash only. You don't wait around for trash to die, you rely on the bubbles killing them off-screen. If you're not constantly moving forward and gathering more mobs, you won't be able to clear the rift. Concentrating on killing a pack at a time doesn't work (unless it's huge density).
You drop bubbles and have them do most of the damage on the mobs. The other skills you use are to proc meteors or reset your cooldown on the slow time bubbles (4pc Delsere's bonus).
Weapons you ideally want an ancient Aether Walker as your main. This lets you cube The Furnace and focus a little more effort on elite packs. AW is generally a must as you use Teleport Calamity to stun mobs and proc the Arcane meteor type, so you always need this up. If you don't have an ancient Aether Walker, you're pretty much stuck using it cubed and then you have some options, such as Gesture of Orpheus for more Slow Time bubbles (since again, these deal most of your damage).
It's a lot of effort and takes a lot of gear to do GR55+. This is why everyone is generally moving to Vyr's this season. Ancient Chantodo's set seems easier to get than an ancient Aether Walker. Archon, while somewhat similar playstyle, is just easier to manage and does just as much (if not more) damage.
DelRasha seems superior for group play, though nobody wants wizards for groups anyway, so it's kind of a moot point.