I would go with a 9" barrel. The blackout round was designed to burn its powder in 9 inches and one of the biggest problems it addressed was the problem of short barrels not burning the powder with the 5.56 round, so why would you want to bring that problem back? Also, ammo and load data designed for a 6" barrel is and will likely always be almost non-existent.
Instead of a spring/buffer, I'd look at the JP silent capture device. It replaces the spring and buffer as a single piece and allows the gun to cycle more quietly.
For your suppressor, the AAC sdn-6 is probably going to have the best length/sound reduction ratio, but if you can deal with a few more inches of added length in exchange for noticeably better sound reduction, I suggest the liberty freedom. Its 10 inches long but I've heard it shot next to a sdn-6 on the same gun and its quieter. Its also cheaper and Liberty's customer service is second to none.
Also, if you will always be shooting suppressed, you can save $200 by having the suppressor permanently mounted on your barrel to bring the OAL to 16", that way you only need to pay 1 tax stamp.
The biggest problem right now is getting subsonics to cycle. If you hand load, its easy with A1680, but that powder is impossible to find right now. Without 1680, you are stuck with factory subsonics that cost over a dollar a round, or modifying your rifle to shoot the subs with other powders which usually means shooting supers will now put extra strain on your gun.