Not really a technical challenge. You could force an equivalent of POSIX, maybe based on WASM, that smartphone makers would be forced to support if they want to

Not really a technical challenge. You could force an equivalent of POSIX, maybe based on WASM, that smartphone makers would be forced to support if they want to sell in your region, and forced to support side loading those apps, as no government should be subject to the caprices of the various app stores. At this point smartphones have a fairly mature feature set to expose to the app. That would also help with the anti-competitive app store practices.