It is difficult, yes.
My understanding as far as engraving and when you have to have one (not an expert, not a lawyer):
- if you intend to sell and the lower receiver already has a manufacturer and serial number (in the case of a non 80% lower, so preexisting pistol or rifle, or stripped lower if sticking to AR examples)
- if you build from scratch and you are the manufacturer, so there is no manufacturer/serial number (80% lower, building a suppressor)
And you can have all the parts but you must be able to build a legal configuration.
In my case, I bought a stripped lower (so already has a manufacturer), I got it engraved, not going to sell, but I found conflicting information when I initially did research. No harm in getting it engraved though other than out the cost of engraving and shipping. And maybe it turns out it has to be engraved.
If you already have a rifle, it needs to stay a rifle until you get your stamp.
If you don't have anything and want to build an SBR, build it as a pistol, don't buy a stock, do the paperwork.
Or you can do a Form 4 and buy a pre-built SBR.