New OP, I agree it might be best to start a new thread.
I had a friend ask about what you are looking for but for a pasture board situation. We discussed it and agreed that full care pasture board was the right solution.
I know that’s not what you’re asking-- but here were my thoughts:
So, stalls/pastures/fencing/run-ins are all part of the farm infrastructure. I have mortgage, repairs, seeding, painting, tractor payments. So sure, it makes sense to pay 1/x (x being number of horses) of that value.
But, if you have your own hay/grain/shavings, those need to be separated from "my* stuff, so we’re very clear who’s stuff is who’s. I have limited storage space, and so carving out room for all your stuff is a PITA and probably will have some sort of expense associated with it. Then I have to hope you remember to keep the stuff stocked and stored neatly. Also, if the place feeds hay outside (we do) then there’s no guarantee your horses is eating your hay and mine eating mine. And what if your hay is inferior quality because you’re trying to save a few bucks? Now your horses is eating my good stuff for free.
Then, it actually makes MORE work for me to feed your stuff. My feed room is organized, I have bins for my stuff. Now I have to cram in another bin for your grain. Then drag in shavings, hay, grain bags from a different location. I have to keep track of all your stuff separately and again, likely having to go to a different location to access your hay and shavings (see comment above about storage). That sound easy: just a few feet a few times a day, but that all adds up over the course of a month–and is free to you but adding time to my daily workload.
I think the general stereotype is that self-care/co-op facilities are filled with people trying to save a buck and the rings/barns/pastures are not impeccably managed. What you’re looking for new OP, is more of a modified full board.
I think your best bet is just to find an affordable full care situation. Then you aren’t having to worry about sourcing/stocking hay and shavings. If you’re looking for the requested situation, and you think it’ll be cheaper to buy your own stuff because the BO is adding a high % over cost to pad their profit margin? You’re wrong. My one boarder’s check barely covers his costs. Instead of sacrificing horse care by finding cheaper (and lower quality) hay and grain, pay the extra to let the horse eat the good stuff. Maybe ask the BO if you can work one day a week to help offset costs? Some smaller programs where BO does their own labor might be very amenable to this offer.