You can apply the Group Content Control to the part before your line.
If you want to protect for forms, you need section breaks.
I do not recommend that.


What you want is, if I am understanding.
To insert a timestamp at the end of the document.
Allow the user to type anywhere after the timestamp and not before.
Am I correct?
What do you want to trigger the insertion of the timestamp? Opening the document? A macro or QAT button?