Buckwheat hull mattresses absolutely need a solid surface - slate or mesh won’t work.
My experience, here in the UK, is that they need no special care to keep them aired, beyond an occasional day with the covers thrown right back. They have great internal airflow naturally due to the nature of the hulls and they are not absorbent of moisture unlike natural fibers.
Like you I started with a normal bed and tried to convert it to buckwheat - I started with laying a solid base over the slats (a sheet of heavy plywood). However, I really
should have also added vertical edging to hold the buckwheat bags as they shift and move.