If you give them just water for the first 14 days of 12/12, they'll stretch like crazy and you'll probably lose a bit of flowering potential because of it. I'd just transplant them much sooner and use less nutes. Your plants were very dark by the time you switched, so you can definitely cut back a bit. I try to keep mine just a bit darker than lime green all through veg.
I germinate in containers exactly like the ones you use, then I transplant straight into the final pot after about two weeks (even if I use a 10 gallon container). This makes the roots reach every area of the pot by the time I get to flower. We probably have the same root mass at the same stage, but mine is wide-spread while yours is like a concentrated cylinder in the middle of the pot, after transplanting. The middle of the pot eats a lot, the outer edge eats very little (only new tips of roots in there). Those fresh new tips get damaged by the same amount of nutes. In the bigger pot, those tips consume everything that is outside your root cylinder. In a 10 gallon pot, there's probably about 4-6 inches of soil between the cylinder and the edge of the container; the fresh new root tips suck the moisture out of all that soil. In a 5 gallon pot, it's only 1-2 inches. The outer edge loses humidity faster, resulting in a higher concentration of nutes in a lower concentration of roots. The more soil there is, the more water you use, and the more water you use, the more nutes there is once the water evaporates.
If you try transplanting quicker to the bigger pot, water only the middle of the pot (around the stem) for the first week or two after transplanting. The outer edge will suck the moisture from the middle of the pot and encourage fast root growth. If you water the full pot, you'll extend veg time by a week or two because you'll have a real slow wet/dry cycle for a few weeks.
For the first "complete pot watering", try to get about 25% runoff, that'll flush the extra salt (shouldn't be much of it since you use less nutes).
