If you're hitting 70-80C you should probably consider turning your fans up or making better airflow. I know that theres the rule of thumb that GPU's are capable of 90C comfortable but that is strictly the CORE temperature, does not mean other components are not getting hotter. 3090's are terrible at maintaining temperatures because the memory chips are on both side of the PCB and only passively cooled by the backplate. So, you could be running into thermal throttling and see those spikes. The other thing is you are using task manager, which in course does not properly show usage correctly for hashcat.
I would suggest improving your cooling solutions and bringing the temps to a more stable position and disable any overlocks. Once you have done that you may see more consistent hashrate.
As for the hashrate, 7z has an iteration count applied to it during encryption so the higher the iteration the harder it will be to crack/slower the hash speed will be. So finding a benchmark of say 40,000H/s might not be comparable since it has an iteration of 1000 where you might have an iteration count of 10000 and only getting 4700H/s if that makes sense to you.
I would suggest improving your cooling solutions and bringing the temps to a more stable position and disable any overlocks. Once you have done that you may see more consistent hashrate.
As for the hashrate, 7z has an iteration count applied to it during encryption so the higher the iteration the harder it will be to crack/slower the hash speed will be. So finding a benchmark of say 40,000H/s might not be comparable since it has an iteration of 1000 where you might have an iteration count of 10000 and only getting 4700H/s if that makes sense to you.