How does your camera's source resolution affect the user experience
Here's a quick tutorial by Steve Martin of Ripple Training on why you need a lot more resolution than you'd think when creating 360-degree videos.
The best reasons for posting to Youtube in 8k are that, for the times when people are mousing around your video in Chrome via the "Magic Window" effect (ie, not using a Cardboard HMD), they will see a very high resolution image even when viewing on a large screen and can zoom in with much less loss in image quality. Additionally, many 360 posters recommend that posting in the highest-resolution possible allows the Youtube encoders to do a better job of reducing resolution by oversampling than you can do yourself. I'm not sure if that's 100% true but will continue testing to find out.
Steve has a great tutorial about getting started with Final Cut Pro X 10.4 and 360° videos that I recommend. You can find it here.