As the author of multiple non-fiction books, with royalty agreements, and national distribution, I’m glad you added the section at the end on non-fiction. It is a different animal.
When I finished my first novel I had three people serve as readers, a la Stephen King, and it was incredibly useful, though none were writers. The downside I’ve found with writer’s groups is egos, either insecure or overblown. They can ruin the experience with barely hidden jealousy or the desire to have the group spend an inordinate amount of time on positive feedback for their work. If you can find a group that doesn’t have one of these people, good. I have not. That may be because I have been doing this stuff for a long time.
As for editing, I could not agree more except that I did come to enjoy being edited, both for content and for clean copy. When I started there was not the expectation that a submitted manuscript would be professionally edited. Now there is and it is an investment well made- editors at publishers will immediately see the difference, and it will help keep you out of the slush pile!