So I have no idea how to multiquote so I am hoping I address everything.
First, there is no way to know exactly how much hay she eats. She absolutely cannot be put in a separate paddock. 5 horses get 140lbs of hay per day. That is 28 per horse. Give or take some pounds depending how they share. She is not a pasture horse and will not ever go out on fresh grass. That is due to pasture availability and the fact that it is full.
She will not eat alfalfa cubes. She spits them out but will happily eat the pellets. I had the cubes mashed into a soup then mixed in with her normal grain and she would carefully spit out the cubes.
Daily dewormer will not be changed that is by the vet.
She only gets a total of 5lbs per day of feed other than hay. Not a huge part of her diet.
I watch her protein intake and if her pee start smelling funky things will be changed. There were stall board horses this summer that when I cleaned the stalls my eyes would burn from the amonia smell. That horse was peeing out everything that was going in.
I do plan on doing endurance with her come spring so technically yes I am trying to feed a horse that needs high energy. Yes she is almost 20 but so far she hasn't been stiff yet.
The only reason I considered the smart pak was the senior grass pellets which is a vitamin mineral supplement because it was the only one I have found so far that balanced everything out the first time without having to add an ingredient here and there. If they sold that in bulk I would just buy a huge tub and measure myself. I have a pretty elaborate set up with scales and measuring cups as that is what I do now.