You could cover the top of each pot with something (e.g. paper/plastic/cardboard) - preferably white - and it would reflect a lot of the heat away from the soil.

My problem is that 1/3 of my pots are without substrates and the heat accumulates in them, I directed 2 fans on my plants but I think it will dry them out. I will try to add pebbles to fill the pots, what do you think?

I feel like you just solved it yourself my guy but if you keep running into trouble I suggest adding co2. Or switching to led. Hope this helps

the temperature outside is 20°. the 2 mouthpieces that we see are open

What temperatures do you have outside? For example, if you have 28⁰ or 29⁰ outside, with the lights on indoors you'll have 31⁰ or more. You just need to put in an air conditioner to lower the temperature. Or simply add a bigger extractor to better circulate the air in your tent. Good luck!