Is the liquid I get after pressure cooking a whole chicken a "complete" stock?
If you are cooking the chickens whole, and I think you are from your description, then your cooking liquid has not had sufficient contact with the bones to extract the available gelatin. You should absolutely be able to make a stock with the carcass later.

As you say, even with the nice gelatinous cooking liquid you have from your first step, you don't have a whole stock without the aromatics. My advice is to make your stock with the carcass as usual, but add your cooking liquid too for a nice boost.