i re-use medium too.. been running same peat-based soil-less medium for nearly a decade now. I do amend it each run w/ fresh myco and good stuff, and every other run or so, i add a little new fresh peat/coir etc + lime, to bulk up what's lost over time..
I reuse medium for a couple of reasons. 1, is to help limit impact on peat bogs. 2. is b/c i use a shit-ton of Vermic and Pearl in my mix. I cant simply toss it in a dumpster or in the woods.. pearlite is dead giveaway. I used to bury it, but after a few runs, it gets too much to bury. Even driving arund town with bags of used soiless mix to take somewhere to bury, is risky and very scary to me.
I shake-off root balls completely and compost those w/ the stalks and leaves, but the medium i got nowhere to throw it away without risking exposure.
I didnt start like this.. just things evolved over time.. started in soil. went to soilless, then dro for years, back to soilless, for a couple of reasons, but one major reason was that it was increasingly more difficult to dispose of used RW slabs and blocks.
RE: mites. i rely on cleanliness (change shoes/pants, block pet access, never go from direct from outdoors to room, etc), ladybugs/lacewigs and neem, no issues. but if u ever get mites, well there is only 1 sure-fire absolute way to get rid of mites. U scrap everything. Trash every single plant. Throw out all meduim. Shut off all yur vent fans. Beethoven the shit out of the empty dark room. wait a few days. Scrub room from top to bottom and all gear incld pots, with h2o3 and a diluted bleach mixture. hang a NPS or two. wait a week. Bomb it again. wait another week, remove yur NPS and then (after 30min or so) try to move new plants in there.. Sprays, Oils, (even neem which i love) are a waste of time and money when u got an infestation. At best you'll simply keep them under control. The amount of time u waste dicking around with mites is counter-productive to say the least. 86 the entire garden and start over. The only thing that get them totally gone, is to remove the food source.