I've found that any time I use store-acquired organic amendments (EWCs especially, but others, too), I'm typically importing fungus gnats, and often thrips, too. Complete annihilation is difficult. Suppression to the point of near-annihilation is much more achievable.
I used Gnatrol years ago to some degree of success (typically found back then in HUGE 2.5-gallon jugs), coupled with the yellow sticky traps, etc. You can also cover with diatomaceous earth as a barrier.
When I went looking to procure another quantity of liquid Gnatrol, I discovered the stuff hadn't been marketed that way in years & was no longer available in liquid suspension solution (Bti), BUT Gnatrol WDG (Bti) -IS- available in powdered form, and available in smaller amounts through a number of outlets that buy the expensive 5-gallon buckets, then sell by the lb. I've been using it with some degree of success, both as a prophylactic for gnat larvae in new mixes, as well as treating existing populations. I mix it a bit strong; maybe .13 oz. or more to a gallon of H2O.
Another member here in Hawaii reports they have effectively used unscented dryer sheets as a barrier, laying them across the tops of pots, per their report to me, but I suspect you'd need to move a lot of dryer sheets come tending time?
For the thrips, having abandoned Azamax, etc., and harsh aerosol sprays, I went to Spinosad/Spinosaids at the advice of numerous others, and that's been about as effective as the Gnatrol WDG has been with the gnats.
HOWEVER, in my research re. Spinosad, there were peripheral/very limited reports of toxicity (to humans? animals? plants?) at higher doses or more frequent applications.
Most of the commercially available Spinosad products come in at a concentration of .05% (+/-) from what I've read. There was a stout product with STRONG concentration listed some place in the forums here a little bit ago, though considering the reading I found re. toxicity, I'd tend to stick with the lesser concentrations.
That's what I have either found from reading, or from experience.
Good luck. If all else fails, there's always anti-aircraft batteries. ;^>)