Update to Android 11 is probably to ensure that all binary blobs dealing with the modem and some other parts are updated. This is done while you boot into Stock (NOT AOSP) Android as distributed by Sony. So, update to Android 11, boot into it. There is no need to register with Google, as far as I know. But you probably should get into the OS fully booted.
When you use AOSP10 based SFOS, the binaries have to be for AOSP10. Do not mix with AOSP11 blobs. The flash script warns for reason as it is the combination that works.