Hmm. I have carried A1, A2, A4 and M4/M4A1, as well as a few boutique models.
My choice would be the A4 plus a good adj stock set up. Keep the ballistics of the 20 inch bbl, get the flexibility of the adj stock and enjoy better performance with better new ammo ( SOST or 855A1).
The Canadians saw this and now use the C7A1-exactly what I described above.
For those who want a larger caliber-okay. but with modern bullet and propellant design, the 556x45 is quite capable nowadays (however, 55 grn ball from a 20” bbl, is still quite a good performer against soft tissue at mid range).
A 6mm or maybe a 6.5 mm in the AR15 platform could be a good upgrade-the 6mm Hagar or 240 Tomahawk pushes 105 grn match bullet at nearly 2900 fs from a 20inch bbl.
Remember, as my tagline states, -no free lunches for the dogs of war.....
I like your proposed set up. I would like to see the 6.8 SPC case stretched out a few more mm then mated with a piston rifle similar to the SCAR-L (with a larger mag well obviously), 18” barrel, and side folding stock.
This would give the advantage of having bullets in the 115 to 135 grain range being driven at a decent velocity. The M-16/M4 magwell restriction really limits the 6.8 from being much of an improvement.