4090 7 GK? Not even the 5090 does that.
It will take 5k 4090 GPUs * 114 days (total of 13.7m gpu hrs) to solve 69 puzzle which is 2^68 keys of work.
At cheap prices of 0.1 usd/gpu/hr it costs 1.37m usd.
I think 0.1 isn't cheap. And also you assume that a 4090 can only do 7 GK/s just because that's the fastest public SW. Anyway, 69 was too risky from the start.