So I believe if you insert a 16" barrel on an AR pistol it's no longer a SBR. If I owned the pistol I'd want one anyway.
In terms of ARs...
If you insert a rifle length barrel, (or a sub 16” barrel with a permanently affixed barrel extension that extends beyond the 16”) onto a pistol lower, you have a long barreled pistol. You are legal.
If you do the same and add a stock, you have created a rifle. You are good to go by the letter of the law.
If/when pistol arm braces are considered a “butt stock” the same above applies. Add 16+ inch barrel and “stock” you are legal.
You can switch back and forth between a rifle and pistol configuration. But, remember you could be pushing the envelope as having all the parts on hand for creating an SBR is known as constructive possession. If they want to push hard enough, you
could be subject to charges in this realm. I wouldn’t see it as a stand alone charge that survives the courts, unless you are flaunting the law with it.
All this applies to any lower receiver that has never been transferred as a “rifle” on a 4473, rather only as a “pistol” or “other”.
If your lower was transferred on a 4473 as a rifle, at any point in its life, or designated as such by the manufacturer, then you cannot convert to a pistol at any point. Once a rifle always a rifle.
Easiest thing is to just toss out the $200 and SBR your lower and go wild... plus an SBR is way more fun than some lame ass pistol (when used as a pistol and not a “shoulder fired pistol” which is basically an SBR and what is catching the ire)