I prefer spring water for my soil needs, because of its mineral content. Many times using filtered distilled or RO water, calcium/magnesium supplements need to be given because it is not coming in via the water supply. If you are not running organic living soil and therefore dealing with active microbe populations, you don't even have to do anything special as far as the water goes... tap water, complete with chlorine will not hurt a thing.
It sounds like you have a good plan, and I would add the FF trio of solubles to your arsenal too. What is frustrating you about it? I will advise you that in that rich soil, you should not actually need to supply nutrients for quite some time, especially if you are planning to successively uppot into larger containers at least a couple of times, putting fresh new soil in periodically with those transplants. While a healthy and thriving plant can take some veg nutrients and get even bigger, you should not actually "need" nutrients until sometime after the final transplant when you have been either been vegging so long that the soil is starting to get weak or when you are ready to go to flower, and the plant's nutritional needs change significantly away from what the soil can provide.
Transplanting correctly does not cause stress. Having transplanted into such a large container you no longer have control over the watering cycle and you are stuck having to water by instinct. If you water wrong and let water sit in the bottom of your container, your plants will suffer. If you don't water enough, your plants will suffer. Starting small plants in huge containers frustrates me like nothing else in the cannabis world. All I can do is wish you good luck and hopefully you can find someone else who grows like this and follow their advice. The advice that I give regarding watering is not for the method that you have chosen to use.
