The oil pumps on these engines use a gerotor ( spelling ?) style pumps, which do wear at a somewhat higher rate that a conventional gear pump, so I think I would e looking at changing that next. This assumes you have put a mechanical test pressure gauge to confirm the dash gauge readings. I understand that there are aftermarket oil pumps that are higher volume, which may help. Not sure where they are available, perhaps Rockauto or Summit racing.
Good luck Jeff