Lots of greats suggestions and comments! One thing I would add is to make sure your horse has either had hay to nibble on or a small feed of hay pellets before starting work; this will make sure there is something in the stomach to buffer stomach acid. This can help prevent ulcers as well as make it more comfortable for a horse which already has and is being treated for ulcers.
It’s also easy to carry a small plastic bag of hay pellets when on extended trail rides. Alfalfa is a great buffer because of the high calcium but any hay/hay pellet will help.