New to flat CMS - clues on dev time?

I think the main work will and should go into structuring the content into useful fields that an editor can easily fill in and then prepare the forms for the Panel (called blueprints).

Since the HTML templates are there, all you need to do is remove the static stuff and replace with the PHP code that fetches the content from those fields. If you really plan to reuse that stuff as is.

You don’t need GitHub or the command line, nor JSON. You do need to set up some sort of server and the editors need some basic Markdown.

Since you are new to Kirby, you will probably need some more time, an experienced developer could do this in a couple of hours/1 day, I think, provided that the templates and styling can really be reused without changes.

See also this thread: Looking for help with Project Workflow, 1st Time Kirby user