Open source tools may not be as easy to use as commercial ones but they are very powerful if you can work through the issues. In this video I talk about why I've switched from iMovie to Blender and how I solved my audio-video sync problem.

Spoiler: Use Handbrake or similar tool to re-encode video at constant frame rate with no interlace detection (due to grey flashes using GoPro Hero 7 Black as source).