-- we run the hot water before starting the cycle
-- we add a detergent booster
-- we run regular empty loads with a cleaning product
Separately, we've removed and cleaned any parts that can be removed and cleaned, but with the same results.
When the repair guy came out, he commented that the garbage disposal hadn't been properly installed (10+ years ago) and that it might be part of the problem. In theory he fixed that issue, and we assumed that might also have been the problem. But, with it back to spewing food bits everywhere it seems unlikely that was the problem. The level issue you point out is perhaps worth experimenting a bit.
The parallel commenters comment about stuff stuck in the arms could be the issue as well