Can't lock down a fields in PDF/A

I'm trying to creat editable PDFs so the user can then lock the fields and send the uneditable PDF/A.

I'm using InDesign to create the PDF.

After filling the form and saving as a PDF/A, it flattens all the fields, but there's still an editable field on top of the previous text. This makes the PDF look terrible with slightly misaligned text overlaying the original.

I just want my client to be able to lock down these PDF certificates after editing them without having to go through the "Prepare a form" menu.

We used to be able to "Print to PDF," but can't do that on MacOS, for some reason.

How can I instruct my client to lock down their editable PDF?