300 gallons seems way high for 1,000 miles, my car gets like 30 mpg so that'd be 33.

Keeps moisture trapped down low depending on your base and drainage, which is where mold starts in the first place. Sand works fine until it doesnt, and when it goes bad you get a rock hard crust that reeks way worse than any shaving pile. Your setup might just drain better than most, so dont expect the same luck everywhere.

Losing five people in just over a year is a lot, and I'm wondering if it happened all at once or kind of dribbled out one person at a time. Did you try anything to keep the group going, like switching the night to a weekend or moving to once a month instead of every other Friday? The reason I ask is that sometimes the schedule is the real killer, not the people. Once you know which one it was, it's easier to figure out if you should rebuild with new folks or just enjoy the small group you've got left.