Finally I got around to asking Audacity to add an option to export the labels track as a chapters json!
With so many people, I'm sure, using Audacity to edit podcast episodes, it's very strange to me that this is not possible yet. Could that be one of the reasons why so few podcasts does use this very useful feature?
If you would like to be able to do this too, I'd appreciate a thumbs up on this issue: https://github.com/audacity/audacity/issues/7078