Wait, Texas doesn't allow you to sell an ADU? Would have thought anything goes over there since my friends so frequently tout ultimate freedom/etc.
"Selling" your ADU would either require dividing up the parcel (which I am guessing is nontrivial in most places) or somehow itemizing improvements on the parcel so that the ADU owner can pay taxes for their unit separately from the main home on the property.
No reason to figure out a solution to any of this when housing isn't much of a problem there.