Humidity is high. 80-85Rh if i dont run two dehumidifier. When i turn them on i cant go below 50. Im 5-6week if flowering

PH does not directly affect humidity. Did i read your question right though you have 70 plants in that 3x3m??? If yes.. this is why. plants breath and create humidity. and need a lot of airflow and exchange. However if the air your pulling into the tent is around the same RH. its not going to get much lower. My ambient right now is around 50%. in winder it drops down to around 20-30% and in summer it can be anywhere from 60-80%, Air condition cools and dehumidifies the air, adding two other dehumidifiers into the mix is a lot of energy. I think you might just need to increase the extractor output or add a 2nd one into the tent or maybe change its filter if it has one should the other temps and RH be ok outside the tent. I also would recommend venting outside vs back into the same room if you can.

If smell is not a problem get rid of the filter, reduces extraction capabilities by like 60-70%. Your in uk I'd guess so more than likely needed.

20-21 is 69F. The air does not have that much moisture hoding capacity at 69F. 70 plants in a 3x3 during flower Is going to leave a huge impact at nights cellular respiration happens at night, plants expel water all night long the more carbohydrates they make. Increase your daytime ambient to 80-83f. Air will be able to hold more moisture. Going to need to setup a negative pressure passive. Should be ok with lights on its nights you need extra air flow to assist with transpiration. Water in roots is dictated by water in stems, water in stems is dictated by water in leaves, water in leaves is dictated by water in air. If air becomes humid the plant can no longer transpire and production at night will cease. Each leaf at night spits out tiny moisture bubbles if there is no air to take that pocket of moisture away then production for that leaf will cease. Faster you can get rid of moisture the faster you can uptake more water with more nutrients and faster you can cool. You are extremely under equipped for 70 plants. I'll be very surprised if they survive the humidity that long.

80 is too high you might get bud rot buddy its warm and moist air that l;little nasties like to hide including nasty bacteria like molds and fungus you could ruin your whole crop more air flow better extraction or de humidifier I have 2 in my room :) them are your 3 options :)