Hi there! Very cute App! I have two myself!
I live in Florida where sand is always an issue, even when grass is present. Five years ago I came across a study from the university of kentucky that found it took something like 3/4 cup of water from a horses digestive system to digest and move 1 cup of grain. They suggested soaking feed in water and making it soupy. More water than grain type mix. This works for me. I have not had sand issues or even gas colic in those past five years! I do feed my hay from nets, but they always end up picking through the ground for whatever it is they can find.
My suggestions would be to follow the water addittion in all feedings, keep them on as much hay as they want to avoid the ground picking. If you can offer free choice bundles out in the field it it most like actual grazing for them, as they will migrate from pile to pile.
I have not supplemented my feed program with any psyllium or oils, no bran mashes or the like. I feed a soaked alfalfa cube/beet pulp/quality mix chaf type soaked to soup. They love it! My gelding gets this look in his eyey when he drinks in the flavored water. Lol good luck!