My first (ambitious) grow

Day 7-9: Had a good environment overall, although the light was probably much too high. I didn't realise until quite late that it should have been 60cm above the seedling, and I had it around 120cm at first, and then 80cm. As a result the stem is a bit stringy. Day 9: I lowered the light to 60cm above the plant, and it won't go any lower. the recommended for this light is 30cm above the canopy, according to this very useful article: https://www.growweedeasy.com/how-far-grow-lights, but it's still in seedling stage and I'm guessing the plant will grow up a bit more, and I can always lower a little bit more. Also it's pretty damn hot, so I don't want to overdo it on the lights. I've been keeping the water consistent and just topped up a little on recommended nutrients (more grow, a little micro, adding a little bloom as it comes closer to flowering). The reason for this is because I want to maintain the consistency of environment and monitor the plant for any signs of unwellness. Because Sativas are sensitive to nutrient burn, I'm being conservative in this respect. The temperature with the light lowered is a steady 26 C, and I'm maintaining the humidity at around 70%. This is pretty good for this strain, as a sativa it likes higher temps and humidity. The water pH is around 5.9, also desirable. I've also been wondering if the plant is getting enough CO2. Is it necessary to buy a measurement device and some CO2 bags? Overall the plant seems very happy and is looking healthy. Day 10: cleaned the inside, added more grow, added more clay pellets since she looks a bit thin and stalky and needs extra support, and there were not many pellets present. Light distance is currently around 60cm. Though it won't go any lower with the straps, I will look at lowering it more with some extra rope as the ideal height will eventually be 30cm for this 250W HPS light. Added more Ph buffer yesterday evening and we're currently sitting around 5.7 ph. I was aiming for 5.8 but this is still acceptable. Temperature is around 25 - 26 C, humidity is around 70%, though I will begin lowering this as she grows. I will probably replace the water sometime this week. Upon further investigation I realised that my water levels were too high, and were possibly suffocating the stem. I reduced water levels by a few cm since I had previously been topping up without realising possible damage. I feel like today is the day that this seedling is finally becoming a plant. She has grown some long roots already. Day 11: I further lowered the light to around 40cm using some extra string. It's not super ideal, but for now it's ok. The plant was falling over it's own weight so I added some extra pellets around it for support and it seems to work. I'm hoping she won't stretch as much now that the light is lowered. The inside fan is still on and on full power to build up extra stem strength and keep temperature down. I'm attempting to keep humidity around 60%, a little lower than before. The temperature is significantly higher with the light lowered, easily maintained by periodically opening the door and leaving in cool air from outside which runs just past the intake fan. I'm slightly concerned about the Monstera Deliciosa plant which is sitting just opposite and above the intake fan. Would this have any effects, positive or negative, on the grow? Google is coming back blank on this.