There is no manual for Lumion.The developers of Lumion (Ferry, Arthur and Remko) want Lumion to be so easy to use that a manual should not be necessary.
Twinmotion got something Lumion has not in my opinion: A good UI.
You're right, the workflow and user interface need to be overhauled even further, so that no function is hidden.Lumion is in a constant state of flux, so the moment you start to write a manual, it will be outdated by the next update. Which reminds me that I need to update the Tips, Tricks and Shortcuts thread, so it matches the 2.2.1 feature set. I'm really looking forward to the day when Lumion is so easy to use that this thread will no longer be needed Video tutorials would also help - I hope there will be time to add those soon!