As he said above - you must get an actual gauge on it.
I would add that fully synthetic can be a bit OTT for an old engine - I don't know the miles or use etc.
The Oil Light switch is only to alert for low pressure not level/volume.
As you took a load of rubbish out the old pump it looks like some damage has occurred to bearings so clearances are bigger so pressure is reduced and so on.
Brace yourself for bad news
