Temperature Question

I have the same issue, mine gets to 22 on a very good day, most of the time it's at 15/16 and as low as 7c at night WITH THE LIGHTS ON 😭😭 My plants do see to have suffered a little bit when compared to others but over all they seem alright :)

You may want to lower RH to achieve optimal VPD but at this stage of growth its OK. You will need a very arid environment (at 20C) later though. ie Flowering at 20C needs 25-40% RH

It's OK. I struggle with the same problem especially this time of year. It's not ideal but as long as it doesn't go below 15c it will be fine

I like growing warmer so for me 21c is a good night time temperature. I shoot for 27-29c when the lights are on. I don't think there is anything wrong with 21c as your max temp but your plants might grow a little more slowly. Maybe a issue for auto's but probably not for a photo where you have control of the time schedule.

You're good at that temp. I wouldn't add any supplemental heat unless your temps are dipping below 10C. Honestly, I would add a HPS light instead of a heater. Those puppies put out the heat and your plants will like the extra light.