The short, hard truth is: The console’s security relies on "efuses" (one-time programmable fuses) that blow with each major update. Once you are on 9.60, reverting to 9.00 via a standard USB stick is impossible.
When you updated to 9.60, the previous firmware was moved to the inactive slot. how to downgrade ps4 9.60 to 9.00
Before you grab a soldering iron, consider these options: The short, hard truth is: The console’s security