So how can nutrient burn or light burn still happen? What’s the point of the CO2 then?

From what I know and understand about adding supplemental CO2, you need to have everything locked in and on point before it's worth it. When this is done it allowed you to go above a DLI of 40 and temps that are higher assuming you keep up with the water needs of the plant. Granted more water in does not always mean more nutrients needed in excess levels, if its taken in a lot of water it likely needs less feed as its drinking so much. Overall best to start a diary so people know a bit more about it all. and look up your lights PPFD maps and calculate your DLI, I bet you could go above the 50% setting.

No diary. So, with co2. You need the temperature to also be at 28-29c for that extra co2 and ppf to push harder and take in more nutrients (as a result of the extra photosynthesis going on). Temperature Co2 Light Nutrients Ventilation And airflow over the stomata vents. (all need to be working as one) You plant is clawing. So, the extra co2 isn't working. If your co2 stays at 1000 and isn't rapidly dropping every 20-30m back to where it's at normal level of 4-450 then THE plant is not making the most of it..