I agree, but not directly the cause. The ferts themselves are not killing the shrimp, but the constant & drastic change/swings in the water parameters from the ferts and large water changes.
You have a 35ppm TDS change in a week, whereas in my tanks for comparison, there would be no more than 10 TDS change in a week. Of course my tank is mainly low light plants like mosses and liverworts. My high tech tanks with lush plants on CO2 only have fish in them.
It would be a better idea setting up a small 10L tank with nothing in there but substrate and mosses. Go back to basics. Don't compromise the shrimp's health for lush plants. Besides, you'll be able to see the shrimps better in a low tech, low light tank with less plants.
Good luck with your next steps.