It's only an SBR when you have a short barrel on it. Don't sweat it.

FPNI

only an SBR when built into that configuration or you have the parts to convert it to that configuration under your control.

for example, Form 1'd lower with 16" barrel with you on a hunting trip, 10.5" upper sitting in your safe at home, NOT an SBR

however, Form 1'd lower with 16" barrel with you on a hunting trip, 10.5" upper in your trunk or gun case, SBR

and another, Form 1'd lower with 16" barrel with you on a hunting trip, 10.5" upper on an AR15 pistol lower also with you, NOT an SBR, as you have two legally configured title 1 firearms.

and yet another, Form 1'd lower with 16" barrel with you on a hunting trip, buddy offers to buy it from you after shooting it, so long as he is eligible to own a firearm (and your local/state law allows FTF sales), you can sell it to him on the spot as a Title 1 firearm. ATF simply asks that you notify them that it is no longer under NFA purview which it is in yours and your friends best interest to do. Once the record is updated, your friend will be required to fill out his own Form 1 with $200 tax if he plans on SBRing it in the future

You can also ship your NFA firearm to any FFL to have work done on it, either with or without a Form 5.

Without a Form 5, just include a copy of your approved Form 1, and a letter authorizing the FFL to have it to perfom the requested work.

An FFL does not need a SOT to work on NFA firearms, just to SELL, MANUFACTURE or IMPORT them.