Avoid using cloud services as much as possible, self host FOSS alternatives instead.

FOSS firmware for printers when👀

Yes to the last bit of edit.

No, because federated.

You don’t need same app on both for phone to use matrix, there are multiple apps that can communicate over matrix, some may not be available on both iOS and Android.

But if you were to use matrix, I’d recommend using Element which just happens to be available on iOS and Android.