Both the numeric results and the large variation of the 140ppm and 80ppm nitrate(NO3) readings don't make sense with 3 30% water changes per week, assuming deep gravel cleans (or surface sand cleans) were performed with each of these. What are your tap readings for ammonia, nitrite(NO2), pH and nitrate(NO3)? Is the tap water adding anything when it comes in?
You have extremely low GH and pretty low KH. The fairly low KH combined with high NO3 could be a contributor or a reason for the pH being as low as it is.
From a technical standpoint the tank looks way overstocked for a 9G/34L but it sounds like you are aware of this and trying to do something about it. I'll leave it to some of our better experts on stocking to comment more if that is needed.
~~waterdrop~~