YOu are correct that the “highly potent facility” guide is not applicable here…if the product is truly non-highly-potent.
Pressure cascade protects 2 things. (1) The product and (2) The environment. When designing pressure cascade ask yourself both questions - do I need to protect the environment? and do I need to protect the product?
You stated that the product is OSD and non-potent. So that means you probably don’t need to protect the environment. Thus, Annex 3 of WHO TRS 957, 2010 is not perfectly applicable here.
Since this is a drug, you almost 100% need to project the product. So your air should flow away from the product, and you should isolate any activity which could cause a lot of particulate generation.
Because of this your cleanest area should be the primary packaging area. This is your product contact packaging, correct?
Also, you should isolate the secondary packaging area, where there might be cardboard, or other board, and paper inserts, etc.
Thus I would have the highest pressure in your primary packaging room, your lowest pressure in the secondary packaging room, and your medium pressure in the common corridor. In this way air will flow from the primary packaging room, to the clean room, and finally to the secondary packaging room.
Alternatively you could keep the common corridor and the secondary packaging room at the same pressure and use a “bubble” or a “sink” for an airlock. But this isn’t as good at having a cascade between the two areas.