My initial thoughts on reading the OP's post were similar to Blondie and Beth Sarim.
Firstly, it really sounds like they have one of those fridge magnet word games to assemble convention themes and they just dip into a bag and pick a couple of them at random. Many of the themes for decades have had the same vaguely "spiritual" sounding word pairings, which are concise enough to trip off the tongue, but generic enough to fit whatever you want into them - especially pre-1996, as you can see from this page which lists all the convention titles in reverse order back to the 1940s:
https://rmovideo.blogspot.com/2017/05/all-jw-convention-themes-through-years.html
It's also interesting to note that while "Pure Worship" hasn't been used before, the word "Pure" was there in 1990 ("Pure Language") and "Worship" in 1951 (""Clean Worship").
Now the question is: if the worship was "clean" in 1951, and the spiritual language was "pure" in 1990, how come they were so different from each other, and that the worship of today is even more different from the teaching and worship in either of those years? Was it cleaner then? Or was it actually less "pure" then, and only clean now? What say you, Lett, Splane, et al?