I am still interested to hear back from the OP. I suspect that the main problem really is that they are not feeding enough hay.
The OP says that they have a number of horses of different sizes and needs, such that they are tailoring their menus to every different horse. That makes it surprising that all the horses are losing significant weight on this diet.
I would totally understand it if they said, for instance, “the pony and the Paint are thriving on this diet, but we have a young OTTB that really seems to need an extra boost.” In that case, the solution probably would be: get in a cool-cal or high-oil feed, or a bunch of vegetable oil, for the athlete.
But having (a) such a variety of horses that they can’t specify a typical menu and (b) having all the horses lose weight, suggests that they are not feeding enough hay.
BTW, feeding basic ingredients is significantly cheaper than feeding a manufactured feed where I live.
The price per pound in cents is: hay 21, oats 23.5, beet pulp 27, alfalfa cubes 28,5. And I buy an extruded feed, to use as treats for clicker trick training. The price is 44 cents a pound. That seems about typical; it is a “maintenance” feed I chose for the size of the kibbles.
I do blow my savings, though, by feeding a vitamin/mineral supplement that costs $45 a month.
None of the American brands under discussion here are sold where I live, though we have a full range of manufactured feeds from several local mills.
It was interesting to read in a previous thread that manufactured feeds in the USA could be cheaper than whole oats, and that whole oats were even hard to find in some places. I would imagine that the feed industry, like the human food industry, benefits from the huge volume of scale in US agriculture, and the subsidies for corn and soy that lower their costs. Also, it turns out that the ethanol fuel industry is a bit producer of distiller’s grain byproducts (who knew?) and so there would be more and more of this available.
When I was Googling around, much of the information and research on feeds focuses on cattle: how to get milk production up, and meat pounds on. But cattle have much shorter lives than horses do, and much lower standards for soundness. If you are going to butcher at two years, you aren’t worried about whether you will need to inject the hocks at 17.