i think it is lacking some major element, maybe P and K

i had similar problems with bad pH also. you've probably locked something up.

When you water what is the colour of the water running out. Yellow? Like urine or darker than urine. That’s what I got and it’s a salt build up.

soak the plants in pHed water (6.5) for 20min,i like to pick up the pots and let the water drain out. then put em back in the water and let em resoak it in.
Is the colour of this water running out darker than normally. Test its pH. Has it changed?

i soak em real good then i flush them with 1/2 strength fish emulsion (with good pH also) this should hopefully fix your problem. it has done wonders for me every time i do it. you should see results (darker new growth) within two days.
it is also very low stress

I use fish emulsion because it’s organic so it is gentle on your plants. Contains N P and K so it’ll hook them up with whatever then need.

make sure the water is the same temp as the grow room so it dosn't shock them

Red/purple stems are common to N,P and K so it's not that helpful.
the dead spots are common to K but slower growth is more of a P.

i had very similar conditions and this has worked well for me.