Yeah we learned a lot for next year. Hopefully we won't have as much crud on it as we do this year.For next season, try to vacuum to waste as much as you can when you open the pool - this really helped me last few seasons. This season, we got lazy (plus the water level was low so didn't wait to fill it back up to vacuum) and took a bit longer than usual (and more liquid chlorine, like 15+ gallons).
In addition, I had to clean the DE filter after 4th days of opening and the filter media / DE was basically GREEN (this was expected as I go through the same process every years). Make sure your filter is clean as well (not sure how to can do that with sand filter though). Currently my CYA is low and trying to get that up....
Our filter is brand new. We did do a deep clean on it the other day when we lowered the water levels since had ran some crud through it.