You will very likely have better water for the fish if you forget about the pH and do regular water changes. Those don't have to be 50% changes to be good for the fish.
If you use the EI fertilizing method you can cut the dosages in half, watch the plants for a couple of weeks to see if they grow less well. If not, reduce the dosages by another 25% or so, and watch for another couple of weeks or so. Eventually you should see plant growth problems. Then just go back to the dosages where you didn't have problems. You might find that you need 1/2 the recommended dose of phosphate, 2/3 of the dose of nitrates, all of the dose of trace elements, etc. In other words, don't assume the ratios in EI are set in concrete.
Now, with the lower dosages you could go much longer between water changes, or you could change perhaps 20-25% of the water every week or two.
A problem that will come up is that as the plants grow, increasing their mass they will need more nutrients, and when you do a big pruning they will need less for awhile. So, you will have to watch them carefully to keep up with their needs. Doing regular pruning to control how much plant mass you have will reduce that problem.