I created a save converter for Super Mario Galaxy 2 since I wanted to carry over my incomplete Wii save to my Switch. I haven't tested it extensively, only on my own saves, but it works great so far.
There are a lot of changes between the Wii & Switch save data formats, so I had to re-implement an entire parser/writer for the custom save format, which was way more work than I thought. Because of that, it only supports Galaxy 2. I don't know if/when I'll implement Galaxy 1 support because that's still a lot of work, and I personally don't need a converter for that game. But maybe I'll look into it over the next few weeks.
It's open-sourced here if someone wants to help out with that:
https://github.com/timschneeb/galaxy-save-converter (btw, the save format of Galaxy 1
was already documented here by someone else, only the Wii version though).
The save converter is available as a web app here: https://galaxy.0001002.xyz/
It can also convert from Switch back to Wii format; as well as from/to a JSON text file, which might be useful for editing save files manually.