Hi Leopold
One approach that you may have already tried is to use one of the many available off-the-shelf HTML to Markdown conversion tool to convert TiddlyWiki’s plain HTML output into Markdown. For example:
There are many tools available; the most flexible are command line based.
The trick to getting TiddlyWiki’s HTML output is to use the browser developer tools to locate the parent node and then right click and choose “Copy as HTML”. If you can arrange for all your tiddlers to be open you might be able to convert the entire corpus in one operation.
Best wishes
Jeremy.