For me, add ons, mods go into two categories: essentials and enhancements (upgrades).
Mission essentials: Sling, Optics.
I concur about the BFG sling. I have the VCAS on two of my ar's. However, for shorter ar like 7.5" I've been using a single point /2 pt setup with magpul sling. Maybe its ergonomics, but thats my preference.
For BUIS Optics, I've used ARMS40, MBUS, TROY. Once sighted in, they all do their job. So for me, it doesn't really matter. For RDS, COMPm2 and T1. The comp m2 is old school for some, but it works. For holosite, I had an eotech that pooched, so I got really good at using it as a ghost ring. For magnified optics, TA01 (NSN?). I really don't like those flip to side for RDS - just another thing to snag on the sling.
Enhancements:
Truth be told, I've went all the way to the dark side and back full circle to almost OEM everything. Other than MOE Grip and stock, and rails, all else is standard a2 style. I think that even rails are an enhancement, why? because I've used zapstrap to secure whitelight to the rifle before (cause thats what the binrats had on hand without having to go through NSN supply chain), and it was more secure than an ARMS mount to rails. The MOE stock isn't proving to be any more lighter or reliable or easy to manipulate compared to the standard 6 pos collapsable stock. It doesn't add any balancing improvements to a 14.5 or 7.5. So I will probably just go back to the standard stock. If I was playing with a 18"+, I'd probably opt for a stock that would help to balance the load and provide better ergonomics for the type of shooting most likely seen with a length like that - that is...rested prone or perched.
In terms of foregrip, I have a Tango Down, Larue, AFG, and the jury is still out whether they make any significant improvements in my shooting skillz. If anything, they just act as an index.
Now, I am mixing a bit of work-related situations with training or recreational situations, but I guess for me KISS to just the bare essential add ons, and then enhancements to fit the situation you're in.
Note that on the other side of the world, ppl using the other very very popular gun are shooting quite effectively without a metric ton of enhancements, other than paracord or boot lace slings. (I'm talking about the trained good guys of course).