Fundamentally estates, how ever they are owned, have to be managed and this includes investment and maintenance.
Of course, It's his private estate, so the estate/him, should fund that.
You cannot change the past but as you say now the two charities get most of the BV money. Surely any improvement to property owned by the charities benefits the charity not the estate.
The Jubilee Trust is a 'charity' that pays for maintenance to assets of the Duchy, using Bv monies. Do you consider that to be an actual charitable expense? Why do you think public money should be used on his private properties? Including to upgrade them and turn them into, among other things, luxury accommodation that can charge rent. That rent does not go back to the Jubilee Trust, as the properties are still owned by the Duchy, not the charity. He is using BV monies to maintain his private estate.
The housing trust was given BV money, and then used to buy properties from the Duchy, so the Duchy then gets to keep that BV money, invest it etc as part of a private estate, and the 'charity' is now responsible for maintaining the properties she formally was, and they do so using BV money. This is the argument, that they claimed it was all going to charity, but they are primarily now using it to benefit their private estate for which they receive enormous income from.