I had the same problem. solution I found is easy 🙂 Start with a new config and mark only the default calendar first and synchronize. This writes the default calendar as default in any inifile. In the second run you activate the other calendars as well, the default stays as wanted as default.
15 min max to do, so no need to find a way to do that afterwards. Just delete the old davdroid-account and start from beginning.
have fun and sorted calendars 🙂