The biggest items that contribute to the accuracy of any AR-platform rifle are (VERY generally): a) a quality match trigger that is crisp, clean, and consistent (regardless of the total weight of pull, type, etc.); b) a quality barrel capable of delivering accurate performance in and of itself; c) a quality free-floated handguard/rail system; and d) a shooter that can competently operate the rifle/carbine (most often overlooked portion of the accuracy equation).
For your 6920...those things will all have to be addressed in order for you to get the most out of the carbine in terms of accuracy potential.
So...first things first...the ACOG. While BDC's have their place, they will ONLY work under an extremely specific set of conditions, including but not limited to muzzle velocity (the biggie), a specific ammo at a specific MV and under environmental conditions (elevation, heat, humidity, etc.). Furthermore, they are also based on the specific mount (the TA51 with respect to your ACOG which you have since changed out to the ADM), height over bore, etc.
With that in mind, the TA11F is based on a 14.5" barreled M4 Carbine firing M855 (62gr) traveling at a MV of 2,970fps with a G1 BC of 0.304 and a height over bore of 2.5" which is achieved with the TA51 mount on an M4 flattop receiver. Now, what specific ammo/bullet/MV/etc. you are running (at least running the majority of the time) will ultimately determine how close (or how far off) the TA11F BDC will be for you at any given range along the BDC scale. Barrel length is but a small part of the equation so I wouldn't be set on an 18" barrel unless that just happens to give you the "combo" of factors you need to get closest to the BDC as per Trijicon (not knowing your ammo or any other pertinent specifics, its a guess as to what may/may not work well for you). The differences between what your rifle is spitting out versus what the BDC gives you can be anywhere from very close (and inch or so) to a good long ways off.
For "precision" work...I'd gladly skip the ACOG and opt for a low-powered, variable magnification (1-4x or 1-6x) standard rifle optic with a quality mil-based or moa-based reticle (just match the reticle to the scope's adjustment type) that you can use hold-overs on if you really must or otherwise dial your elevation adjustments to get on target.
As to the barrel...I wouldn't get to caught up on a specific length, especially a longer, 18" barrel. While 18" is a great all-around barrel choice for a 5.56/.223 AR, it is getting LONG for a carbine and will add forward weight, perhaps unnecessarily, to your carbine even if you opt for a lighter weight contour barrel. You can get quality, match barrels in lengths from 14.5" to 18" without too much trouble, so it really all comes down to what you want/need at the end of the day.
As to the FF tube/handguard...the sky is the limit here. Find one you like and that offers features/functions you deem desirable and go for it.
As to triggers...I am a big proponent of the Geissele two-stage triggers which I have used almost exclusively for the last couple years. They offer a ton of different options based on individual shooter requirements, but I would look to the non-adjustable units like the SSA or even the SSA-E (lighter overall weight of pull) or SD-E for your purposes. If you'd prefer a single-stage trigger, there are a lot of quality options, including drop-in units like those from Timney and Wilson Combat's TTU (both single and two-stage available), as well as JP Enterprises.
At some point in time, you may be better served just building a complete upper to work with your 6920 lower and keep the 6920 upper "as is". Your money...your rifle...your call.
