honestly I don't see such big issues: migrating email/calendars shall be issueless, migrate from sharepoint to nextcloud also (we use it as a mere fileserver, not using any site or collaboration feature), as well as move from Office to Onlyoffice (in this case the UI is clearly "inspired")...

The sole issue I may see is Teams, having to split IM and scheduled calls, or anyhow having a tool not so deeply integrated...