On a Mac the primary operation of that key is to access Help. The Insert [Ins] operation is secondary. The macOS will not permit remapping the Help key even if including a modifier key.
As you've found, a different combination can be assigned to toggle Overtype Mode. Another option is to add Overtype to the Status Bar for a clickable method.
Although Apple's Keyboard Viewer suggests that DaveM121's solution will work in reality it doesn't seem to have any effect here. On my Apple Wireless kybd fn+return/fn+enter doesn't alter the behavior, it just enters a standard hard return. Neither is it documented in Apple's listing of Mac keyboard Shortcuts... there is nothing listed for fn+return.