I run auto irrigation DTW Coco with the halo rings and started out with a similar setup to yours. I found that with 2 plants in flower I needed to mix a 20 litre bucket pretty much everyday. By my third grow I had upgraded to a 100 litre tote for my res to feed 4 plants.
You gotta keep an eye on the run off as the plants get bigger. If you are not getting at least your 20 percent run off everyday the EC in the coco can shoot up pretty quickly and cause you headaches.
pH rising could be just the nutrients. Some brands are more prone to swings than others. I dropped out one product that was doing this really bad and now I hardly even check it. pH swings from 5.8 to 6.4 are healthy for the grow and nothing to worry about but anything outside of that needs to be countered.
Watch the res temps. If it gets too warm it doesn't get properly air rated and is bad for the root system.