How do I watermark a PDF Form using Python?

Question

I need to fill a PDF form with some data, and then apply a customized watermark to the PDF. Currently, I fill the PDF form using PyPDF, then save it and finally apply the watermark onto the PDF.

I attempted using PyPDF's indicated method of watermarking (here):

from pathlib import Path
from typing import Union, Literal, List

from PyPDF2 import PdfWriter, PdfReader


def watermark(
    content_pdf: Path,
    stamp_pdf: Path,
    pdf_result: Path,
    page_indices: Union[Literal["ALL"], List[int]] = "ALL",
):
    reader = PdfReader(content_pdf)
    if page_indices == "ALL":
        page_indices = list(range(0, len(reader.pages)))

    writer = PdfWriter()
    for index in page_indices:
        content_page = reader.pages[index]
        mediabox = content_page.mediabox

        # You need to load it again, as the last time it was overwritten
        reader_stamp = PdfReader(stamp_pdf)
        image_page = reader_stamp.pages[0]

        image_page.merge_page(content_page)
        image_page.mediabox = mediabox
        writer.add_page(image_page)

    with open(pdf_result, "wb") as fp:
        writer.write(fp)

The expected result would be the filled out form with a watermark applied. Instead, I get a watermarked PDF, which has lost all form fields.