I am always on the lookout for programatic PDF tools. I have heard for a very long time that PDF is dying and yet it is still far from dead. Pdfcpu is a newish tool that works on the command line. I was interested in it as it has a simple mechanism for filling in PDF forms. It uses a JSON file to provide the form data, and that along with the original PDF is then used to create a new filled in PDF.
Use the form export sub-command once to get the JSON structure
pdfcpu form export in.pdf out.json
This JSON file would be used to create a programatic template. Edit or generate the JSON with the form data you want. Now create the filled in PDF.
pdfcpu form fill in.pdf in.json out.pdf
What got me particually interested in it is that it is written in Go, which I am currently learning, and is open sourced. Hopefully, someone is thinking about turning form filling into a lightweight, locally running HTTP service.