Like others have said, a course maybe but actually just a few 'stable management' lessons from your instructor would see you right. I have sharers for my horses and I actually a thousand times prefer someone who admits to not having a lot of knowledge and who will listen and do what I say and accept guidance, than the kind of dangerous fool who thinks they know it all and will do their own thing. Caveat that we're on a busy yard where there are always staff and other owners around to ask for help, rather than an isolated/quiet DIY yard where you'd be on your own, I would def go for the former kind of set-up as your first share, ideally where horse is on full or part livery so your job will largely be to ride rather than do a lot of care of the horse.
To pick an example from your OP, re feed, obviously it's nice if someone understands the theory behind what horses eat and why, when you would increase or decrease or change the type of feed etc., but actually in practice I 100% decide what my horses have and the most I ever want a sharer to do is give them a pre-prepared bowl of food or haynet, I don't really need any input from them so if they don't know the difference between oats and grass cubes it's no skin off my nose. Likewise on BHS stage 1 you learn the fundamentals of things like different tack options/bits, different rugs, paddock management etc where again, 99% of the time as a sharer all you need to do is follow what you've been told by the owner.
The things I would say you do need to know how to do:
-Tack a horse up (with basic saddle, bridle and boots, if share horse wears something unusual owner will show you how to put it on and fit it) - spot any dangerous wear and tear to the tack.
-Groom a horse ready to ride including picking out feet and checking for stones (again just basics, you don't need to know how to strap or turn out for a show or anything fancy)
-Put on and take off rugs
-Catch in and turn out a quiet horse safely. Know how to tie up a horse safely on the yard.
-Muck out (this isn't really a skilled job per se but it takes a bit of practice to do quickly and not waste bedding) and do basic stable checks ie does horse have water, forage.
-How to do basic field checks i.e. check water, ragwort or other poisonous plant check, scan for any fencing damage and what to do if any issues (99% of time this is report to yard manager or member of staff rather than try to fix yourself!)
-What to do with a hot/sweaty horse after exercise and how this differs in summer/winter.
-General yard etiquette ie general yard safety and security, clean up after yourself and your horse, say where you're going if you are going out hacking alone.
-Important one, how to spot signs of illness/injury and what to do - although once again 99% of the time the answer is find an experienced person and ask for help! However experienced they are I don't really want my sharers trying to treat my horse if it's injured or making decisions about whether to call vet or not, as that's for me as owner to do (or in an emergency, the yard manager), all I want them to do is spot the issue and escalate appropriately!
I reckon I could teach you all that in a couple of hours, a lot of it you probably already know or half know anyway and a lot is common sense. You can get yourself a book for reference (it's old fashioned but I still refer to my Pony Club Manual of Horsemanship for things I need to look up) and like I say, the most important thing is to be honest with owner and find a set up where there's plenty of help and guidance available. Good luck!