Lauren, I’d be asking myself why my horse was uncomfortable on that kind of surface. That footing shouldn’t be a problem so you might want to look at it as a symptom of a problem and start searching for what it could mean. IOW, what are sore feet a symptom of (as long as the nothing has changed about the feet themselves–when a horse who is sound barefoot goes sore on easy footing, I’d want to know what else is going on). I’m not a vet, but it would be prudent to look into it. If you can eliminate the trim and hoof quality as the culprit, look for metabolic answers.
To give you an example, my barefoot horse was fine booted on all surfaces and did great barefoot on sand, stone dust, and softer footing. Then he got sore even in his boots. Turns out he suffered a bout of laminits during a bad leg abscess. He needed appropriate after care.