I've not read through the thread, but read the OP.
Horses vary, but our foundation here is just good hay, and complete feed pellets.
If they need weight, then maybe a senior complete feed vs. an all purpose complete feed. If they are thin over the back, that is something you can get good results with using beet pulp, BUT you can also usually make it better and keep it balanced using a good senior complete feed that has beet pulp in it.
KISS (Keep It Simple Stupid -- and I know you aren't). I probably spend as much time thinking about what my husband and I will eat, as I do thinking about what our horses will eat, but it all comes back to KISS. It doesn't have to be complicated to work, and the more you can give them time to chew (quality hay), especially with other horses, the happier they are.
Also, I don't think you can get a better coat conditioner than flax seed you buy whole (even Walmart sells it, but if you have a feed co-op, you can buy it for pennies on the dime) and grind the seed yourself with a cheep coffee grinder. It brings out the bloom, and helps horses shed their coats. It also enhances gold tones. Inexpensive, but super healthy (and not just for them, but it's good for people, too!). If you feel like it, you can go on and grind a bunch of it and put it into containers then put them in the freezer, or buy the seed in bulk and keep them cool (or freeze whole) and grind them periodically. It's so easy, and the best "feed-thru" coat conditioner I have used.
Best of luck! Hope to see you posting pictures and show wins