Welcome to the field. Please take this advice from a white-haired old engineer. The best of source of technical information and guidance is available directly from the manufacturers and their distributors. All the major manufacturers have application engineers and local distributors. Their only job is to help you successfully choose and apply their products. It has amazed me over the last several years how few young engineers take advantage of these services. I guess if you grew up with Google as your only source of general or technical information you might not think of these other sources.
So call a couple of the major manufacturers and ask for an application engineer. Talk directly to a live person.
Also, confirm that the current valve is indeed the model you think it is. If the manufacturer's information says it is a 5/2, its a 5/2. Just because you can stop a handle in the middle of it's travel does not mean it is designed for that.
Your description is unclear. Are you saying that the piston has air on one side and extrusion material on the other? Or is the extrusion device separate from and driven by the cylinder? A pneumatic diagram would help.
If you intend to operate this cylinder at this low pressure, you should be prepared for some trouble. All cylinders have internal friction, and this friction is a major issue when operating at low pressures. The resulting action will be jerky and uncontrollable. Redesign your system so that higher pressures are required.
The only reason I can see for a valve to have a minimum internal differential pressure is if there is an internal pilot valve. If you redesign for higher pressures, that won't be an issue.