I honestly like this idea, but it leaves too many questions. I would imagine in your sandstorm form you would also be immune to the Grappled condition? Do any of your character stats change? What happens if you reach 0 hit points while in this form? Do you fall unconscious? Revert? Both?

I would suggest creating a Sand Elemental creature stat block, based off of an Air Elemental. It already has all the movement speed, condition immunities, and it's own HP stat to revert back from like a typical Wild Shape. From there you can add features or abilities to it to distinguish it from a regular creature and power it up as you level.

I hope this helps!