I use Adobe Premiere so my options may be different than what you have, but I almost always export in 4K (even if my timeline sequence and/or recording resolution is lower. Usually I use 1080p). I always use maximum depth and quality.
What seems to make the biggest difference though, is the bit rate. For just fun family videos that I just want to get posted to share with family and friends, I'll use one pass and a bit rate of 40 at 4K. If it's a project for someone else, I use two pass with a target of 40 but a maximum of 68. If I rendered the export as 1080p I use 10 as my bit rate for the target, and if it's two pass, I use the max at 12.
YouTube will automatically re-render video to a lower bit rate which can have terrible effects on videos. Since the Hero6 can record a bit rate up to 78, you lose a lot of information in the pixels when you upload to YouTube. By exporting and then uploading as 4K, your able to use a higher bit rate.
These are the bit rates used by YouTube.
2160p (4k) 35-45 Mbps 53-68 Mbps
1440p (2k) 16 Mbps 24 Mbps
1080p 8 Mbps 12 Mbps
720p 5 Mbps 7.5 Mbps
