I do not know anything about Kindle Direct publishing, but it does not sound like a serious publisher. Kindle produces eBook readers, and Kindle's eBook format is not suitable for scientific books. Lambert Group is a network of predatory publishers, and I have seen horrible books (compiled simply from Wikipedia articles, or prints of Google scans of old books, etc.). See also https://www.researchgate.net/post/Is-LAP-LAMBERT-Academic-Publishing-is-an-authentic-publisher .
i think both these ways which you have mentioned either KDP or LAP are not worth to publish our research ....i think there are always reputed journals...book publishers and other where we can follow the format and publish our research for which we have spent our lot of energy to get better output or profit...