How is your Spinach doing and what hydroponic method are your using? Your questions are complicated, and answers may be long but I'll try to keep it short and brief. , It's nothing like growing outdoors in soil and requires additional knowledge to learn to be successful in hydroponics. To be successful It's best to have some knowledge of plant and hydroponic science, and not just how to put a system together out of a box.

For a perpetual growing system you desire it will require 3 separate hydroponic tables. After you seedlings sprout in plugs and set true leaves they are tranfered to the first table for the first vegetate stage circulating a predetermined lower solution concentration. Once established they are transferred to the table best for what you're growing. One for leafy crops an the other for tomatoes. Changing the ratio of nitrogen in the last week before harvest would reduce chlorophyll production to reduce bitterness, so I don't know how you'd accomplish this without effecting other plants not ready for picking. withholding any nutrient would not apply to tomato plants because you don't eat it's leaves and the plant will continue to grow to harvest the next cluster and so on.

To address your other question, plant nutriment requirements will change over it's life span but depends solely on what you're growing. Leafy crops like spinach or lettuce are never allowed to go into flowering or produce seed so these type plants will have the same nutrient ratios from start to finish on the 2nd table. Crops producing flowers and fruit (seeds) will have a different ratio to support the nutrient demand when they flower and set fruit. BTW, indeterminate tomatoes will grow for years and each single vine will produce a cluster of flowers every 21 days so I don't understand the need to replant every 4 to 6 weeks, and the process is completely different than spinach.

There isn't a premixed nutrient recipe which fits all. Lettuce and brassica like spinach need more nitrogen than phosphorous and potassium, while fruiting plants need a different recipe during it's different stages.

Plants have an ability to compensate moderate high or low nutrients levels through the growth of their roots and transport process and somewhat forgiving, but are not able to compensate for the wrong ratios of all the nutrients so the recipe is critical. Once a recipe is zeroed in the solution quantity can be refined using a EC meter to check the solution. The electrical conductivity of the solution represents the total dissolved solids and using some mathematical formulas can translate the EC numbers to total parts per million, but does not indicate a lack of or excess of any specific nutrient and only the total. So if the ratio of the recipe is right and the EC number is low or high then the quantity can be raised or lowered. Since you studied this subject for years I'm sure your aware of water PH but suggest diving into the science behind the process to be successful. Good luck