So I've had a 14.5" 308AR for five years now that I put together myself; Black Rain billet set, 14.5" BA carbine barrel, DD Lite Rail, Syrac adjustable carbine gas, and AAC 51T mount/suppressor. It's OK, but I'm just not feeling it like I think I should be. Keeping it running without adjustment going from unsuppressed to supressed is hit or miss; it seems either undergassed without the silencer or way overgassed with it... and it's fairly finicky when it gets dirty. I've tried a 14.5" midlength on it and the dwell was just too short to get running well. Slash heavy buffer, lighter buffers, different springs, tubbs carrier weight, PRI fat gas tube, regular gas tubes, two different high quality BCGs, etc...

I'm thinking of either migrating to something else entirely or maybe changing out the barrel assembly on the AR for a 12.5" and trying to make it 100% reliable.

Metrics in order of importance to me:
Suppressor Use - Easily accepts the use of a suppressor without lots of tinkering.
Reliability - Both suppressed and unsupressed.
Longevity - It's getting an SBR stamp, so I'd like it to be very robust.
Optics Options - I'd like to reasonably be able to put a LPV or CQB type optic on it easily
Accuracy - Being an SBR, it's probably only going to ever shoot <500yds... so anything under 2MOA is fine.
Weight - I'm not too hopeful any are going to too light, lol.
User Servicability - How hard are they to work on from home?

I'm leaning towards the 11" or 13" SA58, but I know very little about FAL type rifles and they are a tad bit pricey. The PTR is about $1100ish. And I could probably get away with modifying my 308 AR for <$400.

DSA SA58 11" Pistol with added Para Folding Stock and 51T Brake
https://www.dsarms.com/p-16595-dsa-sa58-fal-pistol-11-barrel.aspx

DSA SA58 13" Para Elite with long FH swapped for a 51T Blackout
https://www.dsarms.com/p-12491-dsa-sa58-16-para-elite-edition-rifle-pinned-barrel.aspx

PTR K3P PDW 12" Pistol with added HK telescoping or HK fixed stock and 51T Brake
https://www.atlanticfirearms.com/component/virtuemart/shipping-pistols/ptr-k3p-pdw-308-pistol-w-paddle-mag-release-detail.html?Itemid=0

Keep my Black Rain 308AR Billet set. Swap 14.5" barrel out for a 12.5" Ballistic Advantage. Install a suppressor-friendly adjustable gas setup or, if available, some kind of piston conversion(?). Maybe even drop a LaRue, HiperFire, or Geisselle trigger group into it to breathe some life into it. My current 14.5" 308 setup and what it might look like with a 12.5" barrel:
Attached File

If you're going for a new stamp I say scar 17 with 13 inch barrel. Mk17 clone

Maybe I'm in the minority opinion, but I think the the SCARs are ass-ugly and feel chunky as hell.
They're not heavy but I don't like the way they handle for me. And $50 magazines...??
They're solid performing weapons, but I wouldn't buy a SCAR even if they were $1500.

(I feel pretty much the same about the SCAR-L and the FS2000.)

wouldnt waste that money on a 308 sbr, lose too much energy for the cost.

Get a 762x39 sbr

Inch for inch, given the same weight bullet, the 308 is still going to outrun the 7.62x39 by at least 200-250 FPS, I’d guess. Plus it’s way easier to find good 308 ammo.”, even if it is pricier.

I had an SA58 and I sold it to buy a SCAR 17.  It's the only gun I've ever sold and it was totally worth it (to me; just saying).  I love the shit out of my SCAR 17.
DSA's quality is questionable, and possible issues with their rifles aside, their magazines are utterly useless.  You talk about SCAR magazine prices (which are are more like -$40, not $50) but DSA's FAL mags are complete shit.  If I wanted good magazines for my SA58, I had to buy surplus because (unless things have changed) there arent any good new production FAL magazines out there and I was spending close to what a SCAR 17 mag costs for a good condition surplus FAL mag.

I'm not trying to talk you into buying a SCAR 17 or anything, if you dont want one you dont want one.  If anything I'm more saying I would personally suggest staying away from DSA.  From what Ive gathered their quality hasnt exactly improved...

I voted to modify your current .308 AR.

For my  $.02, I would steer away from a gas operated SBR in 7.62x51.  The port pressure would likely be very high, and add a suppressor it increases even more.  Now this may or may not be an issue for durability and reliability, but I'd tend to go with the PTR91, being a roller-locked mechanism.

PTR-K3P with a fixed stock. I absolutely love mine. Just waiting on my stamp!

I once read something about having to change the angles on the rollers or something if shooting a lot of suppressed to delay lockup more... true? Is roller lock complicated?

PTR-91 G3K Clone Build by 556 Channel (HD), on Flickr