The are problem many problems NFA item, besides the fact that the feds know you have it. For one you can't take it out of your state without letting the feds know that you are taking it out of the state and getting an approval from them. Second is when you want to sell it, it becomes a mess if you do not want to sell it to an FFL, and forgo getting top dollar selling it to an individual at retail. If you want to sell it to an individual, there is a TON of paperwork through the ATF, and the gun cannot be transferred to the new owner until the paperwork comes in which can be over a year. It is a mess for you to try and hold someone's money for months, and maybe well over a year, while you wait for the paperwork to clear. The other issue here with the NFA is that many states do not allow NFA items, so if you have a braced pistol and you want to register it as an SBR, you just may not be able to do it in certain states.
If you have a braced pistol make sure that your state allows NFA items before you even think of sending the ATF a photo with a form 1 online. Remember that you are now telling the ATF that you have an NFA item, that has a 10 year prison term and $250,000 fine for "possessing it" and that it is illegal in your state no matter what. You could end up in a big mess, never trust these people.
If you do not want to register it, I believe that the best solution is to remove the brace and get the brace out of your home. If there is no brace, there is no SBR. Also remember that it is legal to convert a pistol into a rifle, but not the other way around. So, if you have a braced AR pistol you can just throw a 16" barrel or upper on it and you are good to go, you can also add a regular stock on it AFTER you have put a 16" barrel or upper on it.
If you always wanted an SBR, well now is you chance, register it with the brace, and AFTER your paperwork comes in, you can put a regular stock on it, and you just got a free SBR.