My recommendation bro is ditch that fox farm if u haven't already got it yet and get some your soil from buildasoil.com best quality organic soil u can buy period!
As far as reusing soil this can definitely be done successfully however I do recommend a few things a good quality compost and worm castings at a 1:1 ratio with a sprinkling of kelp meal and crab meal, no need to remove the roots u can pull a plant out and plant directly back into it I would cut the old stem as long as u can before the top dress and it will cover it up the roots will still have exudates that will help your new starts
I always agree with the use of myco fungi can I ask which one you use currently??