Oh, I'm so interested in the full rabbit hole list. I was not even aware that Spy Kids are a Grindhouse spinoff! Then again, I think I watched it only once in my life with half a brain (left)....
In any case, I am curious about the full in-universe canon + spinoffs list.
Okay so, Grindhouse introduces Machete as a fake trailer starring Danny Trejo, which then became a real movie. Danny Trejo plays the uncle in Spy Kids, and for whatever reason Robert Rodriguez decided to imply that both characters are actually the same character.
As for the rest of the rabbithole list, I'm working on it. It's really up to personal interpretation as to what is canon though.
Tarantino himself has stated that there are two separate universes for his films. Films such as Reservoir Dogs and Pulp Fiction are considered part of the "Real" universe, while films like Kill Bill are part of the "Movie" universe that are just fictional movies within the Real universe.
From Dusk Till Dawn is part of the Movie universe, so its sequels are too. However, FDTD features a reference to Desperado; someone uses the crotch gun in FDTD, which we see briefly in Desperado. Does this mean that Desperado is part of the Movie universe too? What if Desperado is Real, and FDTD simply features a Real weapon? Once you go far enough down the rabbithole, things get confusing, there's no confirmation for what counts as what. For the time being, I'm just going to disregard the universes when talking about tangential films, and treat everything as ambiguous canon.
I might also be disregarding the Elmore Leonard universe because I don't know much about it.