well, i won't really advocate reusing the potting mix as a whole, unless it was in the isolated 'clean' pot and free of disease. then adding humus and extra bark or perlite should be ok. i've done that for like petunias or pansies, smth not demanding and disposable. the rest can go into compost pile (which of course i don't have in the city).
but, of course, i'm mostly indoors gardener and put a lot of my indoors plants on the balcony just for summer. so they stand mixed up with permanently outside pots - and can get contaminated with some eggs/larvae - so i have to be very careful with my mixes and up-keep.
i deff reuse scoria after rinsing - it takes 5-7 years for it to fall apart and it has nutrients too.
and you can 'disinfect' the bark by pouring lots of boiling water thru it.
i mean, i agree that some large seeds will germinate well in moist bark. just not small seeds.
google 'mesh seed trays' or 'netted seed trays' - i think those will be just right, turn them upside down and drape wicking mat on top. and then they have to go into 'water trays' and your seed-trays on top. kinda elaborate. some hydro-tray set-ups might be easier.
when i started with my suggestions i did not realize you have a large 'production' going on.
i do very few seeds. only smth special. i sprouted zamia furfuracea and lignum vitea just for fun, with a heating pad and moist perlite under dome (zamia took 4 months!). will see if they can survive the winter. but it's fun.
