Suspect you can go even shorter than that of the overall length after P&W is at least 16”. Believe you can even permanently affix a suppressor to a shorty and aside from the tax stamp for the can, technically it’s a rifle now and no need for a two-stamper
Most muzzle devices from Dead Air and SilencerCo will take it over 16". With a longer muzzle device you could go with a shorter barrel but then more velocity is lost.
I personally couldn't care less about a free stamp. I'd prefer my guns to be braced pistols and so I will wait out challenges. IF I decide to add any SBR's to my trust down the road, I'll do it when I'm good and ready and pay for my own stamp.