Do overwatering not at every watering then, and dose the fertilizer according with the runoff EC when you do overwater. It's really the root zone EC that matters mostly, and runoff EC will be a good aproximation. More frequent flushing will give a more stable nutrient solution (especially in terms of nutrient balance), there's a reason they grow in rockwool in top greenhouses. Also, as long as aeration is high (coarse peat is especially important, not sure what promix has), you can have high water retention, the roots don't rot from water but from lack of oxygen.
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