I’m in the same process as you, and have started threads much like this myself in times past. @JB is a lot of help in this area!
I seem to scale back or change things up and then slowly start letting things creep up, adding this and that for various reasons. My gelding is an easy keeper, has skin and allergy issues in the hot months and has anhidrosis. He’s also barefoot and needs support for his hooves, which aren’t bad, but that’s because I am so particular with his diet.
You need to start with the absolute best quality hay and enough of it. If you board, this may be somewhat out of your hands, but you can always “supplement” with bagged hay (chopped, cubed, pelleted) if needed. Get that 1-2% of their body weight covered with the best forage you can and the rest comes down to filling in the gaps and targeting any conditions the horse may have.
The easiest way to do this is via a ration balancer (fed in 1-2 pounds per day) or a forage balancer (fed in ounces with a carrier like hay pellets or beet pulp). Either of these options should cover the horse’s nutritional needs. If you board and have to go with the barn’s feed, that’s fine, just make sure the horse is getting the minimum requirement listed on the bag to fulfill its nutritional needs. If the horse can’t due to weight issues, etc. add a forage balancer or ration balancer and that should take care of everything.
If you’ve done the above, the horse has everything it needs nutritionally. The next area a lot of people look at is “condition” (weight and hair coat) which often can be improved with the addition of a fat sources. Flax products like Omega Horseshine and many others are popular as well as rice bran products and various oils. Easy-keepers won’t need as much fat, but adding a small amount can help with coat, etc.
With all of the above covered, all you’re really left with is any specific issue the particular horse may have. Joint supplement, digestive supplement, respiratory supplement, etc. These will be things you may want to discuss with your vet to decide if it’s a matter of adding a supplement or treating some other way.
As for the company? It doesn’t really matter. SmartPak is a retail company that sells all sorts of things including supplements. They have their own brand as well as many other brands. Platinum Performance is overpriced, IMO. That said, I’ve just ordered some of their Platinum Refresh in a desperate attempt to get my horse sweating (I’ve done literally everything). I’ve used it in years past with some success, so fingers crossed. None of their other stuff is all that great to me.
I like Mad Barn a lot as a company. Their Omneity gets great reviews.
It’s going to come down to what you already feed and what you feel your horse needs in addition to that. Make sure the foundation of the feeding program is good first. 1-2% of high-quality hay, hard feed (whether a fortified feed, ration balancer, or carrier with forage balancer) to fill in the gaps of the hay. If those things are in order, your horse shouldn’t need many (or any) supplements.