I got the Timing belt/Water pump done L&M for $699
While they were in there I got my right Motor Mount Replaced at M cost only $120
I also had the front motor mount replaced
I had The Rear Differential done for $99
The Oil (I know I could have done it for less but I wanted to spend the extra difference of $12 to get a new washer and have them look at the old oil coming out) for $32
The AC vac and refill was $49 (normally $149)
Tires rotated and speed balanced for $39.95
They checked the clearance on the valves, topped my fluids off, washed my car, went through a list of other random questions I had and gave me a loaner car for a grand total of $872
I don't understand. You listed a bunch of things they did, with prices that add up to $1039, but you say your grand total was $872.
To keep my MDX running forever, I feel like I should plan to swap my transmission, power steering, brake and coolant fluids as well as the spark plugs. BUT NOT at dealer prices; each fluid is $129 and the spark plugs are $269
First of all, they almost certainly changed your coolant for you when they replaced your water pump. (If you're not sure, check your receipt or call and ask them about it.) Assuming they did, you won't need to flush the coolant again for another 60K miles or 5 years, whichever comes first.
Intervals for flushing/replacing the transmission and brake fluids and spark plugs (and doing other services), as recommended by Honda/Acura, are noted a few posts above. They don't have to be done at the dealer; any competent independent mechanic can do them. You can even do it yourself if you have the time, inclination, experience, tools, and a place to work on the car. But if you want to keep your MDX running well, you should make sure you get those services done at the recommended intervals, no matter who does them.
I'm a little surprised you couldn't negotiate what you consider a decent price for doing those additional services at the dealer at the same time as the other services you did have done, since there should have been labor savings by doing them then. Note, I'm not saying the service prices you're quoting are or are not a decent price, since I haven't priced them individually. I know the stock plugs themselves, without the labor, are around $100. (If you're comfortable buying cheaper aftermarket plugs, you can do that, but dealers won't usually install them.)