

Nextcloud has a similar file storage like SharePoint/OneDrive minus the content types and taxonomy trees, but I doubt you need those. If you use Only Office as online Office App in Nextcloud, you have a comparable UI to Microsoft and it uses Office Open XML (docx, pptx, xlsx) as standard file system.
I don’t know what a paid hosting for your team would cost, but it could be worth it.
The best thing: German Customs is in the Fediverse. Their post: https://social.bund.de/@Zoll/114851460544943879
Their account: https://social.bund.de/@Zoll
And I think they take questions.