I've tried which I believe is your guys' way; using front matter via -d FILE per
https://pandoc.org/MANUAL.html#general-options

When I use that argument below with a valid data file that pandoc finds, in the GitHub Action run with my custom Docker image (note: which again.. is just the generic pandoc 2.14.0.3 docker image with tlmgr install collection-fontsrecommended applied to it and using default latex converter):

Whose file file contents above ^  contains just this:
---
fontfamily: helvet
...

I get this in the run:

Error parsing pandoc-pdf-config.yaml line 2 column 0:
Unknown option "fontfamily"

Thoughts? Am I doing something wrong?

Thanks for any help!