Hi,
Overlaying super long videos can be cumbersome, so I'd suggest to consider a subset of "interesting" laps. To overlay longer videos - or laps covered by the 2nd or later fragment, you need to join the raw videos using iMovie or Handbrake and transfer it to your phone for overlay. While the GoPro splits videos due to file system limitations, these limits do not exist for your 256 GB super device
Please have look into Video Documentation available on www.gps-laptimer.de/documentation. It includes background information on the way LapTimer aligns data and videos - which may help when working with video sections.
Sorry for the late reply, missed some posts lately...
- Harry