Hello,
As a fellow ROM maker, I have a recommandation: Rather than copy/pasting Google's manifest, use Google's manifest as-is, but include your platform_manifests as local_manifests.
As an example, the way to sync my ROM is:
repo init -u https://android.googlesource.com/platform/manifest -b android-12.0.0_r16
git clone https://github.com/phhusson/treble_manifest/ -b android-12.0 .repo/local_manifests
repo sync
repo will automatically include all manifests in local_manifest
I prefer this way, it feels easier to maintain to me.