(We really need an agree button, I keep on thanking people when I agree with them.)
I actually disagree with the idea of putting a book in KU first. Wide readers are a tough nut, but when they fall for an author they are loyal, and they do not like having to wait to get their books if they don't buy on Amazon.
I suggest a survey for your readers. Write a short story and for those who complete the survey, they get the short as way of thanks. Include different things from how many books they read a week or month to how many they buy? What store would they prefer to buy from? Do they belong to a subscription plan? If so, which one?
If 80% of your newsletter is KU, it's going to be an uphill battle, but the survey will give you a better idea than what you can guess at.
As for readers not needing an explanation, they really don't, so don't give it to them. However, let them know that you've decided to offer your books in other stores and so that means you'll be removing them from KU. Apologize for the inconvenience, and say you hope they'll still with you. Promise that you'll occasionally enroll a book in KU, but it won't be a new release (seriously, can't say this enough, in my experience and research, nothing annoys readers from other stores more than having to wait an extra 3 months to buy a book).
If you're serious about going wide, study the other stores. Look at Apple Books and Barnes and Noble and make your books look as close to those books as possible. There is a huge difference between wide and KU (neither is better than the other), and you need to be aware that Amazon's market is a very different market from the other storefronts.