With good fans, airflow and the correct GPUs, that spacing challenge is easily solved.
At this point, I will do almost anything to get rid the of bloody risers

My group has more than 300 x GPUs (90% with risers) and the root of all problems start with those risers.
I hear you. I imagine you are going to go for blower style GPUs. Putting one big fan behind and all airflow goes the same way.