So there is some disagreement among hydro growers, some run the water up into the net pots, others do not. I am in the latter camp, the one time I ran the water higher got me into trouble. I always run 1" below the bottom of the net pot, even if you don't have roots showing. The surface agitation of the water is enough to get moisture up into that newly forming root zone.
Generally, the more water in your system, the easier it is to stabilize the ph. The problem with those large net pots is in order to keep the water level below the nets, you cannot get as much water in the system. For you, I would say that isn't as much an issue as it was for me in the one plant system.
As far as res changes, I started out changing the res every week but I do believe that is unnecessary and can waste nutrients. I'm still learning, but what I am doing now is changing the res when it needs to be changed by reading the plants. For example, I am running a strain and flipping to flower, so I changed the res for the first time to get the bloom nutrient ratios for the next phase of growth. I would also want to change out if I was reading something with the plant and having a hard time figuring out what is wrong. Changing out the res gets you a "reset" of everything where you know what you have in the water and can eliminate other possible causes of the issue if that makes sense.
The most valuable thing I have learned from Moe and Aqua is to learn to read the plants. I'm still learning HOW to read them, but once you start looking in that direction, you will learn them just the same as me.