1
If it is indicated your Surface device is not up-gradable to Windows 11, are you offered the option to register for the ESU (Extended Security updates) on the updates section in the Settings app?
2
While it is true that Windows 10 is no longer in full support, that should not stop the Office (Microsoft 365) apps working, they should continue to work normally in Windows 10, what problems/error do you see when you try to use Excel?