I use a fan speed controller just as you say to control temp/rhSounds like nothing more than the fan is running so efficiently that it is pulling air from the tent faster than the humidifiers can add enough humidity to show on the tool that is measuring it.
Since you want the air exchange to be pretty much constant to keep the odor down you can consider a speed controller for the fan. That might allow you to slow the fan down to the point where it is moving only 75% of the air allowing for a small increase in humidity. It might be enough to keep the smell at a tolerable level.
Though, not worrying about the humidity any more and letting it be what it is currently might be better for your blood pressure.
In another grow a long time ago I soon realized I was pumping in CO2 then sucking it right out again
Now I let the air circulate a bit more before it gets sucked out by reducing the rate of extraction [is that vpd?]
SMSCOM controller with an RVK centrifugal fan with optional temp probe which I don't use
Best I can find in Euroland