Hi,
after these many beta versions I would like to get rid of everything beta in order to have a clean install of the released game. To do this is it ok to "delete local data" in the Steam Options and reinstall GalCiv III or do I also have to delete some folders manually?
Yes.
This support page has full instructions (about halfway down the page, in the 'Refreshing your entire GalCiv3 Installation' section):
https://esupport.stardock.com/index.php?/Knowledgebase/Article/View/496/161/general-galactic-civilizations-iii-troubleshooting