Keep in mind that it is easy to change an input field with 'readonly' attribute to be editable.
Moreover, showing that value in a text field when user cannot edit it, can be somewhat confusing for the user. Better to show the value as normal text, and include a hidden parameter.
Finally, You cannot depend on the front end to not change the value of a field. If it is supposed to be unmodifiable,
You should make sure of that in the server side code handling the form submission.
HTH