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.

I like Japanese integrity. Yes they acted to late, but at least they chairman „expressed his intention to take responsibility for any harm caused“ and stepped down.