Unfortunately, you'd have to actually make a left and right hand version... 'hand agnostic' wouldn't work with this kind of design.

Imagine a non reversible hand agnostic glove.

Hope your thumb and pinky are identical, on each hand, just swapped.

Mirroring the entire hand isn't the same as a hand being symmetrical.

... you could perhaps try to model a mouse around a pistol grip or flight stick... not sure if that would be doable to work out to the correct wrist angles though?

Hrm.

Actually, a standard dildo with balls/flared base design might actually work as a kind of basic shape to start from...

... the balls would be the place the pinky side of your hand rests on... then you'd need symmetrical flaring/indentation where either thumb could go... make the left and right click and scroll wheel be along the ... would it be dorsal? side of the shaft, have a button or switch elsewhere that can flip button assignment and wheel orientation between left and right handed use. Scroll wheel is horizontal due to grip angle / both handedness.

Then I guess add more bonus buttons the thumb could use.

You'd also have to weight the bottom of this thing to prevent it from toppling, the whole thing would have to be fairly heavy.

And you'd have basically double the amount of buttons as per a particular handed variant...

... design is all about compromises, lol.