Ok, so I don't know if there is an easier way to do this, but I think this is probably one of the few ways to do what you're trying to do. It makes it hard cause of multiselect etc. but this should keep the same formatting and search capabilities and allow multi choice.
You're first step is to create a collection with Person records. So for this, on the onSelect property for your Division Dropdown DataCardValue8 you'll want to do something like this:
ClearCollect(colDivisionAdmins,ForAll(Filter(Admin,Division= DataCardValue8.Selected.Value).StaffAdmin As Adminx, Adminx.StaffAdmin))
This will create a collection of People records from the admin list that is usable in the existing DataCardValue25 combobox, so you can keep the formatting. The ForAll is used to go through each record from the filter table using the Table function to basically copy the "Record" for the StaffAdmin copy out of it and into the collection for use in the Combobox in next step.
Next, unlock the StaffAdmin Card on the CreateItem Form (Under advanced). Then select the DataCardValue25 and on the properties tab make sure Select Multiple and Allow Search is turned on.
Finally for the Items property of the combobox DataCardValue25 plug in colDivisionAdmins.
