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I built my rifle. Lower isn't registered as a rifle.
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true, stripped lowers do not designate one way or the other, but you made it into a rifle first. You might consider reading a little about how the rules on that work. (in short, pistol to rifle is ok, rifle to pistol is a no go, per ATF)
Anyhow, to your topic at hand. I think the parts you have are just fine. Can you do supressors in DE? If so, maybe consider what mounting system you want to use and get that now when you build the upper.
edit;
So, just read a brief bit on DE gun laws. No supressors... that sucks. But it does seem like you can do a SBR. Be safe, SBR the lower and gather the upper parts while you wait. dump the brace and be happy you can run a stock on it once the F1 is approved.