Export your Notion workspace as Markdown & CSV (with subpages) and unzip the archive.
Download script.
curl -O https://github.com/tanrax/notion-to-joplin/raw/main/notion-to-joplin.py
python3 notion-to-joplin.py -f <path/to/your/export>
Example:
python3 notion-to-joplin.py -f b25c8352-f87b-4b5b-ce0a-61d09c5bd81b_Export-9e0c6ec4-762b-4d70-b30e-045ece8b4722.zip
You can now import the folder import to joplin to Joplin (File > Import > MD - Markdown directory).
import to joplin