There are a few requests for pies within pies, but I can understand the reluctance due to recursion (pie within a pie within a pie within….). So my suggestion
Pie Trays
It’s a container that only holds pies, not individual shares or other pie trays. It could be optional (ie turn it on under settings as it doesn’t effect current pie implementation).
It would mean you give each pie a size and when you deposited money into the pie tray it is divided up into each pie and they do their normal investing or holding as cash, etc