Happy to chime in, been watching the thread with great interest. I think the only (possibly a non-issue, as long as you're aware of it) issue you might not have expected is that in a DI system you can expect the semi-auto function to be forfeit with subsonic ammo. Blackout is amazing in that the gas systems are built from the ground up to cycle subsonic ammunition, and the supersonic excels by virtue of always having an excess in terms of what's necessary to operate the system.
Large ARs on the other hand, are designed and built around functionality for supersonic ammunition of varying weights and applications. This makes a transition to subsonic get really interesting, because none of the current factory gas systems are designed with cycling subsonic ammo in mind. This is true of specifications such as gas system length, gas port size, and system inertia metrics like spring strength, spring length, buffer weight, BCG mass, etc. All this becomes much more feasible in a piston system such as what you'd find in LWRC, POF, and GA Precision's large AR systems, but those are off-the-shelf spec systems not intended to be purpose modified, and generally prohibitively expensive in terms of experimenting towards success.
All that said, if you can live with, or build towards the expectation of setting up a system to utilize both super and subsonic with no substantive change to the system, you can do so quite happily with the understanding that subsonic rounds will almost certainly need to be manually cycled. I'll discuss some potential exceptions in a minute.
My rig is a Mega Arms MA-TEN matched set with the monolithic upper, an 18.5" mid-weight profile Titan barrel from Fulton Armory, rifle-length gas system with a .093" gas port and .936" gas block journal, and SLR Sentry adjustable gas block. An Ops Inc .30 cal 12th model suppressor/brake combo completes the muzzle package. I shoot primarily Federal Gold Medal Match 175gr through it, and when suppressed the muzzle report with supersonic ammo is fairly well on par with .22LR from a standard sporting length rifle. Many here on the forum have both shot this rifle and heard it fired and can attest to the accuracy of that description (or inaccuracy if they wish). I've also in recent times had the pleasure of running Ruag (Swiss) 200gr subsonic rounds through it, as well as varying brands of 175gr SMK subsonic, in which case as has already been stated by @98Z5V it's literally "hollywood quiet". The only caveat with subsonic is that it has to be hand-cycled, but very well worth the trade-off for me.
I'll wrap up with my thoughts on what would give you your best chances of successfully cycling subsonic in a large AR:
#1. A standard length barrel with a short gas system. The idea here is to get as much gas into the system as possible before the bullet exits the muzzle.
#2. The largest possible gas port, I think .125" is as large as a gas block will permit.
#3. As little weight/spring pressure as possible in the reciprocating mass of recoil system and BCG. It's gonna suck for supersonic, but that's the price of cycling subsonic.
There ya have it, my thoughts on the matter! Welcome to the forum, and looking forward to learning from what your build ends up teaching us all!