yellowish plant, stunted growth, burnt tips, leafs looking weird. 100% no insects

cannabis can take a high DLI even from a seedling. 40 is about average for what they get in a day. I'd say keep it aounrd 40, could always drop it down a bit if you notice signs of light stress then increase it again later when it bigger. The 5.5 PH is an issue, Its just to low. I would aim for anywhere between 6-6.5. this way if it swings outside of the desired PH range it has buffer room. Right now if it gets lower then 5.5 is really going to put a bummer on the roots. You can increase the PH of the water going into the plant by adding PH up. Potassium hydroxide. Or adding in more calcium or an alkalizing agent into the medium. My last grow in Peat moss and perlite had something wrong with it and it kept dropping in PH uncontrollably. I had to bump my waters PH upto aorund 9.0 for it to normalize. Something, most likely a bacteria was in the medium acidifying everything. Red stems can be a symptom of phosphorus deficiency. This is most likey caused by the PH being so low. http://www.plantsgalore.com/care/images/Nutrient-availability.jpg this chat can give you a bit of a visual of how available plant nutrients are in different PH ranges. Given that you are in a biobizz lght mix "peat based medium" and using Biobizz nutrients I would flush the medium with a high PH to normalize it and then issue a smaller feed after its drained and settled. Use a TDS/EC pen to confirm its concentration is not to high and that the PH is back within range. Given that the PH will most likey swing down again keep an eye on it and adjust as needed. This might be a weekly thing for this grow. Hopefully not. Congrats on nailing the VPD. You should also Calibrate your PH and EC pens to confirm your readings are accurate before proceeding with any troubleshooting. Best Of Luck!