I think you guys never tried a good closed rig with right air pressure. Space between cards is enought if air flow is correct. With many kind of gpus (5600/5700 for example) you can take off fans from the gpus.
Open rig can work only in a small environment.
I'm using 4 Mining rigs with 6x GPUs on each and I'm doing this in my very home, no basement or garage but just inside a room, it's well ventilated and I believe that's all that matters, if one can afford extra coolings nice those cases with fans then it isn't bad either, OP is using RTX3090 which are pretty hotty but it all depends on the temp even if the cards are pretty close to each other in the rig