Persist list of checkbox selections on records with related children

Hi,

I'm having difficulty finding the best approach to updating a list of checkbox options on a parent record using related record associations. Essentially, I am exploring the ability to save multiple selections (child rows) on the "create" of a new record and ability to "update" those persisted selections through an update process. I have a simple example expressed below through the use of local variables for clarity but hoping a few insights will help me understand how to apply at a recordType level with related record data. The content related to post# 22489 was very helpful but not fully understanding how we can add new items and also remove (inactivate the unselected items). The end result would be the use of a checkbox or multipledropdown field where a user can add new rows (isActive: true()) by selecting and unselect saved rows (set isActive: false()) to write back to the record table.

Sample code:

a!localVariables(
  local!supplyList: {
    { id: 1, item: "Pencil" },
    { id: 2, item: "Paper" },
    { id: 3, item: "Binder" },
    
  },
  local!student: {
    id: 1,
    name: "John Smith",
    suppliesNeeded: {},
    
  },
  local!selections,
  {
    a!checkboxField(
      label: "Choose Supplies",
      choiceLabels: index(local!supplyList, "item", null),
      choiceValues: index(local!supplyList, "id", null),
      value: local!selections,
      saveInto: {
        local!selections,
        a!save(
          target: local!student.suppliesNeeded,
          value: a!forEach(
            items: local!selections,
            expression: { 
              id: fv!item,
              isActive: true()
            }
          )
        )
      }
    )
  }
)

Screenshot of example in work: 

Any insight would be gladly appreciated!