I agree entirely with the difficulties of trying to mix the kids library. In much the same way as trying to mix an iTunes and ripped cd library.
But the kids+ is much more than books. If you have an iPad try the kids+ application. On a kindle fire just the joy of having apps that are advert free is such a big thing.
Plus the annual cost is basically free when bought on prime day with a new kindle.
So I bought a kindle kids reader side loaded only the books I wanted on to it. Used the kids+ subscription for the boys existing kindle fires. So they can read the books they want on the subscription. But when it’s school reading time they only have the books that I want them to see.
Though if you need a reason to buy something else I still hate the touch screen interface. I still use my original keyboard kindle for that reason.