Enforce the kernel team must be separate from the application layer - let other people build operating system UIs on top of the kernel.
For the operating system team to be separated from the product teams.
Even go further and unbundle the product teams - make office separate from bing which is separate from edge, etc.
Just make sure you also do it to Apple, Google, etc.
This is what the US almost did in the 90s.
A mere split up will lead to “baby bells” and the bigger one will just buy others - and centralize again.
PS: We should remember that Microsoft was able to destroy Nokia with an installed CEO (Stephen Elop) of their own. Killed the already shipping Linux smartphone. Installed Windows Mobile and Nokia was finally dead. Nokia itself did mistakes before but from outside this was questionable?