I have created a web-based tool to create markdown from KOReader exported JSON.
https://kotomd.utsob.me
The input file/copied JSON in the editor on the left must be valid JSON. You can then copy directly from the pane on the right or download the markdown file.
It also has a settings panel to do some tweaks. You can choose to format highlights based on the marker type you've used while highlighting and select what styles to use for those 4 types of highlights.
It only saves the settings in the browser. No data will be stored anywhere else.