I clicker train too and I can easily go through a cup or two of treats in a session. It has to be cheap, safe enough to feed in volume and importantly not rot in your pocket. Nothing like finding week old baby carrots in your jacket!
I too have ended up using a pelleted complete feed, choosing a brand with a larger kibble. I get the 50 lb bags not the same stuff sold in 5 lb bags of “treats”!
None of the horses get this as daily ration and it is motivating enough.
The exception is one place we go once a year to perform where we work on a grass lawn. There the grass is too tempting and I need to up the great value. So for that venue I use hard white peppermint candies and bulk ginger snap cookies!
Interestingly although my mare is super food oriented she is rather cautious about strange foods and hates anything mushy that sticks to her mouth.
Her eyes about popped out of her head with delight the first time I gave her an over ripe pear. She loves apples and carrots of course. One summer she developed a taste for frozen lemonade slushies but when the weather cooled off she lost interest.