With short flood cycles you can get away with higher flood height. Be careful the cubes aren’t washed around by water. You may wish to pot the cubes in hydroton or clay balls for weight. It also holds more water and air that way.
Depends on your local tap and what you’re using. I recommend Dyna-Gro, 1.5mL/L in veg, 2mL/L in flower. It’s got everything you need and nothing you don’t, in easy 1-part liquid format, inexpensively. If you have soft water, use CalMag with it, or a base nutrient with higher Ca/Mg. I don’t test ppm/EC, even with active hydroponics. Use a big reservoir, change 1/2 the volume of the reservoir every 7 days, and you won’t run into problems.
I learned hydroponics from the aquarium plant hobby. We called it “estimative index dosing”. The concept is that with large, frequent water changes and re-dosing what you estimate to be the appropriate amounts of fertilizer, you never have to worry about any nutrient being lacking or building up.
You mentioned in another thread that your room temperatures were high, like 30C or so, are you able to keep your reservoir cooler than that? Reservoir needs to stay <22C.