Well they are 2x GH5, not the GH5S, so Aux TC is the only option.
For me it is not about using the first few frames or not, it is the fact that the video ends up being out of sync. At least it is a consistent offset.
No it is in sync. Just missing audio for the first 7 frames. If your external sync is taking the start point incorrectly then of course that will be out of sync but the audio in the camera is in sync.