Then invest in and attract people to build it. I'd move to Europe if the salary was competitive.

IMO start by funding the living crap out of open source projects. Mandate that hardware sold in the EU comes with unlocked bootloaders and documentation sufficient to develop drivers from.

Relax IP protections so developers are allowed to reverse engineer products and build derivative works from them (extending the life of, facilitating compatibility).

Ban security systems used by big companies that enforce OS conformity (like kernel based anti-cheat, or banks disabling tap-to-pay on phones running beta android/rooted).

Double down on platform interoperability - e.g. Allow me to write a chat app that uses Facebook messenger as a back end.

Hey-ho there you go, European competitors to Android/iOS will pop up overnight. Asahi Linux and other OSes will get a shot in the arm (ha).