Embrace the NFA. Take the plunge and just be done with it.
I would suggest that if it’s an AR type, that you register as “ALL CALIBERS” on the form to give yourself all potential options...
If you've ever wanted an SBR, this a golden opportunity to get one without paying the $200 tax.
Once your braced pistol has received an SBR tax stamp, you will then be free to take off the brace and put on a proper stock. (But don't do it until you receive the approved eForm 1, otherwise you'll be committing a felony. Only BRACED PISTOLS are covered under the amnesty period.)
Yo-Yo is exactly, right, here is the specific guidance from the ATF:
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the Court also explained that an NFA firearm is made if aggregated parts are in
close proximity such that they: (a) serve no useful purpose other than to make an NFA
firearm (e.g., a receiver, an attachable shoulder stock, and a short barrel);
or (b) convert a complete weapon into an NFA firearm (e.g., a pistol and attachable
shoulder stock, or a long-barreled rifle and attachable short barrel)
—