Everything else is in the same potting media, made of coco chips, coco coir, and perlite with an added biochar.
Seems like a reasonable choice of pot mix components.
The ratio of these is important, in balancing water holding and air pore space.
None are completely inert, they all take up water to various degrees.
Some inert components, like quartz gravel,may give the same structure, but lower saturation.
Generally I stop at 33% coir for my mixes.
( that is the finer grade coirs, also same for other fine grade bog peats }
Most of the methods I mentioned are preventative or suppressive, although they might help recovery.
I would warn about trying everything at once to fix a problem.
Many nursery plants are grown and shipped with a dose of Fungicide to ward off problems, they might go backwards after it wears off.
A fungicide treatment sounds the best first option in your case.
It does sound like a possible problem with cutting raised plants ?