you are not required to deregister to sell. Once the NFA firearm in question is no longer in NFA configuration it is no longer an NFA item. It can be sold without any intervention or involvement with the ATF.
For your own records of you can send notification to the ATF about an item not existing as an NFA regulated item but it’s not required.
And yes make it a pistol again and sell it without involving the ATF. Let someone else form 1 it on their own later.
Edit - and by the strictest interpretation of the law you can’t sell the pistol and SBR kit to the same person before their form 1 is approved, as someone from any number of alphabet soup agencies could argue constructive intent.