If you tested all GPUs - the problem in risers, that you haven`t tested. 850Wt is enough for 4*3070 downvolted GPUs if you are mining ETH or other algorithms with memory load.
And if you use multiple psu's make sure you use the same PSU for the riser and GPU or you can damage your gear!
It does not matter. I use PSUs in different ways and havent seen any problems. And i even cant understand why it can become a problem.