Quick update - all starting to veg better now, though the few weeks of age difference in seedlings and young plants is irritatingly laborious in terms of water/feed/lighting in one small tent...
The mistake I made earlier was using a high K root booster without realizing how much it would raise the pH, even if it reads right when first mixed; plus I sometimes use Citric to adjust pH, which may be weak and break down too quickly
I tested my run off for the first time in my life; watered 5.8 it came out 6.4 - rub my chin emoji
So now I have leaves on all plants I am going to start to up the lights and nutes and push them a bit
This needs a good feed when it dries out a bit and a dose of CaMg/Enzyme/Boost but the roots are solid and bursting out of the bottom of the pot