Still doesn't explain why so little of the Kaula material is available? There's certainly not much obvious similarity between the Spanda or Monist schools and the generation/completion Chakrasavara practices, as far as I can ascertain.
The reason is very little of the vast corpus has been edited and published, and reading manuscripts is extremely hard work. There are only a handful of people in the world who have read broadly enough to get a full sense of the various Śaiva systems. Witness the confusion of terms like "Kashmiri Shaivism," Kaula, Trika, Krama, Spanda, Pratyabhijñā, Monist, Northern vs. Southern Shaivism, etc. Prof. Sanderson's website has a treasure-trove of articles (all required reading, IMO), but unfortunately the book-length article “The Śaiva Exegesis of Kashmir”---which offers the best general outline of the scriptural, ritual, and philosophical classifications used by Kashmiri exegetes---is not available on-line.
An article which is
available, “Swami Lakshman Joo and His Place in the Kashmirian Śaiva Tradition," discusses the state of the living tradition at the time Sanderson began studying in Kashmir in the 1970s, and the reasons for its historical decline.
But yes, Buddhist systems of the two stages such as the Cakrasamvara traditions don't seem to have much in common with any Śaiva system.