the internal properties of PlanItemDelegate depends on styleData.* properties, so if I try to make

But they're already bound.

PlanItemDelegate {
    id: planItemDelegate
    fullMdl: tv1.model
    indx: styleData.index                     //< This is a binding, isn't it?
    closedHeight: mainW.height / 12
    open: styleData.selected
    onButtonMinimizeClicked: {
        root.changeCurrentIndex(-1)
    }
}

What difference does it make if you create the PlanItemDelegate and then pass the reference by id, or just pass the reference. I don't see any difference or problem whatsoever. Perhaps I'm wrong, but it really seems the same to me. The only thing I don't know is where the styleData object is defined ...

PS.
I have now encountered styleData, but am still not sure where it comes from, or rather how it's supposed to be accessed.

indx: styleData.index
closedHeight: mainW.height / 12
open: styleData.selected

...
indx: styleData.index                   
    ...
    open: styleData.selected
...

the internal properties of PlanItemDelegate depends on styleData.* properties, so if I try to make

itemDelegate: planItemDelegate

I can not bind this custom properties. Or Now I think about some js function like

itemDelegate: {var copyObj = planItemDelegate; copyObj.index = Qt.binding(function() {return styleData.index }); copyObj}

And if I reference this object by id, how do I bind delegate specific properties?

I'm not sure I understand the question. What delegate specific properties you want to bind, and to what?
Do you mean something like this:

Text  {
    text: planItemDelegate.closedHeight
}

Do bear in mind, that as I noted my whole experience with QML totals the 2 hours I put in this evening, so I may be completely misleading you ...

itemDelegate: planItemDelegate

PlanItemDelegate {
    id: planItemDelegate
    fullMdl: tv1.model
    indx: styleData.index                   
    closedHeight: mainW.height / 12
    open: styleData.selected
    onButtonMinimizeClicked: {
        root.changeCurrentIndex(-1)
    }
}

rowDelegate: Rectangle {
    height: planItemDelegate.height
    color: "white"                  
}

This is how I anchor things to one another, at least in a very similar matter.

rowDelegate: Rectangle {
                    height: planItemDelegate.height
...

ReferenceError: planItemDelegate is not defined
And this is real trouble to get this property. I have not found approach to get it.

rowDelegate: Rectangle {
    height: planItemDelegate.height
    color: "white"                  
}

just work?

PS.
... assuming the PlanItemDelegate component has that property.