I would prefer to do everything in LED and did not know that about DC. What would I use for that then, an HO transformer? If so, how many would I need?
As far as using the DCS remote for the yard power and uncouplers... I only found out recently (by seeing a post on FB) that this can be done, however... I think relays or something are needed and that is way out of my league. I understand electric to a point, but when getting into relays, capacitors, diodes, resistors, AC/DC, etc., I'm as confused as they come. (I do know that houses are on 120V AC, sometimes 220V, and vehicles are 12V DC..... that type of stuff)
If going with 12V DC as an accessory lighting voltage bus you could use a 12V DC wall-wart, 12V DC power supply adapter, 12V DC from a PC power supply, etc. Hundreds if not thousands of choices. Less than $10 and any of them would power hundreds of LEDs.
But if you're lacking time or aren't comfortable working at the component or module level, there are many suppliers of LED lights and lighting products which can directly connect to the 14V AC Accessory voltage from many train transformers...or to the AC voltage from your KW set to 14V AC (or whatever you choose). These products have the diodes, resistors, etc. built-in to work so you don't have to mess with AC vs. DC, regulated vs. unregulated, Amps vs. mA, and similar jargon tossed about with LEDs and lighting.
As for using the DCS-AIU to control Yard power and/or Uncouplers, who would have thunk that you read about it first on FB rather than OGR! There have been several OGR threads on how to do this and, yes, in some cases you need to add relays, in other cases you don't. So to your point, it can be confusing!
Everyone has their view-of-the-world but as I see it, the electronics used in O gauge depends on what is going on in other more mainstream applications (automotive, PC, communications, etc.). So we pick up the scraps where we can...like 12V DC LED lighting strips. It was a big-win to discover these could be adapted to O-gauge applications. But as you have seen in your other thread, maybe not such a big-win for adapting 120V AC panel meters to O-gauge voltages!
I guess it comes down to what you want to do. And, since you're in the planning stage with somewhat of a blank canvas, I figure now's the time to blue-sky the options. What you don't want is the "V8 moment" where after everything is built and wired, you realize you could have had a fill-in-the-blank....