I like the tape flag idea for meds!
My barn has one color bucket for AM and a different color for PM. All have labels with horse’s name, get made up in the feed room and stacked in order. Fed off the wheelbarrow used for hay in the barn and the outside horses have theirs driven out with their hay.
The barn does not offer soaking. Any horse who needs to have something soaked such as to administer a certain supplement or medication, the boarder feeds that themselves or works with another boarder or their trainer if they can’t be there. I do this for my horse’s Robaxin because he doesn’t get enough grain otherwise to make him want to eat it.
It is kind of a pain for some of the horses with meds and supplements, and we have several horses on pergolide which can’t be taken out of the package too early. But most boarders try to just have one container to dump and not several supplement things. These are on a separate shelf and labeled per horse.
I feed a different grain and supplements so I bring containers with everything mixed. I suppose you could ask your horse owners who feed supplements to make up containers on a weekly basis or something even if they are feeding your provided grain. I try to make things as idiot proof as possible in case of staff changes and also just to cut down on the amount of individual needs one has to keep track of for so many horses.