I wanted to go check my safe, and make sure you didn't somehow get mine off the blackmarket! Then, I realized you said keymod, not M-Lok.
Those M21's are decent enough sights. My beef with them is that when the Tritium decays you're basically stuck buying a whole new sight. I asked Mepro about replacing the ampule, and they said it isn't that simple. The entire internals have to be replaced, and the cost to replace that is only a small savings compared to a new one. Since you already committed to it, go ahead and enjoy it. When it dies, you might consider something else.
Definitely put that hand stop on it. I personally feel like this is the key element in the OEM handguard, that all of the aftermarket handguards are missing.
Since you asked about accessories and mods, my biggest suggestion would be a suppressor, and LOTS of ammo
Put a can on it, and you are going to fall in love with this rifle so much that you'll need that extra ammo
The system is overgassed on purpose, for reliability reasons because it is a military design. That extra gas comes out in a few places. Many people like to seal the "gas leak" spot under the rail, as well as the ejection port cover. It all depends on how much you
love the smell of gunpowder in the morning and when you decide it becomes too much of a good thing. Manticore Arms makes a replacement port cover that includes a gasket, or GearHead has one that includes another QD sling connector if that's to your liking. The spot under the rail is easy to seal up with pennies worth of high temp sealant. Most use RTV or form-a-gasket type silicone. I used a high temp Poly-Urethane because it's what I had laying around.