[Supermario]

With all valves in neutral and flowing back to tank what kind of pressure is in the system? I'm just wondering, some systems use a slow return check valve for the oil returning to tank. It keeps the valve in a more charged and ready state. Using a flow control you could restrict return flow to a point and not generate excessive heat. Let's say you have 50psi in a free flow state right now, using the flow control you could increase that to maybe 75 or 100 psi and see if the functions improve? Also, your prv would need its own dedicated return line to tank and not tee into the valve bank return line.
It may be something to experiment with before going through the hassle of converting to closed centre system?