DD are very nice rifles. I have an MK18 upper on a lower I put together myself that I like a lot. I'd hold off a little while on the brace/stock, until we get some clarity on the legality of the ATF's recent rule change from the courts. If they somehow reinstate the legality of braces, maybe look at the Tailhook or an SB Tactical brace. If you're going to have to SBR it might as well get a full on stock. I also like the B5 stocks, but there are lots of choices.
As for the red dot - Naaman has some good recommendations. I personally like the Trijicon MRO, but many people do not. The chief problem is that they have a significant parallax problem. I shoot enough and don't really shoot in weird/awkward positions, so it's rare that I don't have the dot pretty much centered in the optic - thereby avoiding any parallax issues. But for many this is a deal breaker. As for MRO HD, don't have any personal experience with one - but I think that's Aimpoint T2 money, so you might as well get the Aimpoint.
Eotech's are also nice, have a great reticle and almost zero parallax. They also work really well if you plan to add a magnifier. However, they have shitty battery life, if that's an issue for you.
I personally have recently purchased a couple of Holosun optics which I have been very happy with. They are an excellent value choice.
Agree on a trigger upgrade. LaRue MBT's represent the best value. Geissele's are currently on sale and are arguably a hair better, but even on sale will be a little more money.
If you are in the market for suppressors, I'm a bid Dead Air fan. But a number of places are blowing out KGM suppressors (KGM used to make some of Dead Air's cans). Personally, I'd snag a KGM R30 or R30T from here and never look back -
https://www.bauer-precision.com/suppressors/?_bc_fsnf=1&brand=353. Makes more sense than buying a dedicated 5.56 can, because you can probably use the 30 cal. can on most, if not all, of your bolt guns.