If you would like to use command line scripts to transform Day One JSON to Obsidian Markdown, have a look here:
Being scripts for one task each, I think they should be rather easy to read:
jq flattens the JSON file and creates a single Markdown file with a mix of data for output and data needed for the next steps. Then AWK reads this file, creates filenames, fixes attachments links, and creates one file per Day One entry. A final bash script looks at the outputted
.md files and creates folders of the form
YYYY/YYYY-MM as needed and moves the files.
This works for me, and you can tweak things yourself, too.