I freehand dog rockets
800 grit to 9μ film would be a nice jump. Then on to the 3μ and then the 12k or 1μ film.Personally I don't like combo stones. YMMV
If funds are a problem you could fill in with a few sheets of film for cheap or even 1000 grit W/D, 2000 grit W/D, 3 micron film.
Enthusiasts should still be enthusiastic about their stones, maybe more so.
I like the Naniwa Superstone 8k, but the 3k doesn't really give me the warm fuzzies. It loads up pretty quick. Like @bluesman 7 I don't like combo stones. Anyway the film will fill the gap cheaply and you might even decide you don't need the stones after all. Or wait and see what pops up on BST. Once your gap is filled with film, there is no hurry in filling it with rocks. You might also consider a nice coticule for the midrange. It is a good gateway drug into naturals.