I've noticed with higher temps, tannins release faster, at least with driftwood (explains why boiling removes them quicker).
Sounds like that humic acid is some potent/concentrated stuff. I am afraid it will require removing the substrate and seriously rinsing it (guess you could do that in-tank) or using new substrate material as I think all the powder has saturated the substrate.
What substrate are you using? High CEC?
I bet the pH is crazy low. Have you seen any recommended dosing guide for use humic acid powder?
I've heard Purigen absorbs tannins really well, but with as much there as it sounds, it would probably need to be recharged often. Water changes might be the cheaper option. Stirring the substrate as you drain the tank might help release more to get it out quicker.
Maybe capping the substrate with 3 inches or so compacting substrate might seal off tannins from releasing so quickly into the water column. Though the substrate pH might be really low for plants liking.