Navigating along the long crossing trek

Hello,
I'm starting to plan my next adventure and the Lofoten long crossing seems stunning.
I've read the suggested trail in this awesome web-site and also inspected the GPX files for each phase.

What is not clear to me is how challenging the navigation along the trail?
Base on some you-tubers who shared their experience in Lofoten, it seems like many parts of the trek are not marked, also there is no clear path to go.
Can you clarify that point? Is most of the walk is on a clear path? no path? how challenging the navigation? (I have Garmin device for navigation + a backup map).