Choosing different sets of boards

I would like to create an aggregated view that has one main board as a starting point and a bunch of additional boards from which the data is going to be pulled. A simplified example is showing a graph of all linked items to the current board.

It’s not possible to deduct from the data which board is the main and which is auxiliary. Therefore, I would like to have an option to specify it as a setting for my custom view. However, there is no Board in the settings list.

When I create a custom board view, then in the development page, there is button “Boards” that allows selecting multiple boards. I assume, it is supposed to work as single selection to pass the board Id in the context.

A custom dashboard view allows selecting multiple boards in the settings, however, it passes everything as one list, and not possible to create another selection in the setting for the main board.

As a workaround, I could create a plain string setting where it will be possible to pass the main board ID. But is there a better way to achieve this?