Why is the tag in front of the name 'Sci-Fi'? I don't really think 'sci-fi' when I think of Dragon Ball Z.
Because it technically is Science Fiction. Really unconventional science fiction, by western standards, but just being Japanese doesn't exclude it.
Shouldn't the modern Goku style no longer have the Spirit Bomb as an involved technique, then, given that none of its practitioners have known/used the spirit bomb for generations?
Do the Saiyans know there isn't one, or have they merely not found one?
Hope springs eternal. There's always somebody trying to reinvent it, and they keep this big gaping whole in the style ready to plug in a
really f*cking huge bomb as a finisher. Technically, the style is actually somewhat handicapped by their insistence that they'll figure it out some day.
Well, technically, they don't
know given that it's theoretically possible that the Guardian be squirreled away in the planet's core somewhere, but if that was how I ruled that they know something, I'd say that they live in a permanent state of near-total, blissful ignorance. Going by that criterion, they don't
know that Earth blew up, they just
know that there was a lot of blue light and the feeling of Gohan popping like a high-explosives grape. It could totally be fine, if he somehow managed the literally impossible and directed the explosion. In this case they've searched Garenhuld quite thoroughly, and feel confident in saying that there's no Guardian who happens to be tucked away somewhere. No Lookout to be found, not even when they have a pair of eyes on literally every inch of sky Garenhuld has at the same moment, no outstanding ki signatures, not even when they have the Lords of the time risk going to full power outside of the Hall specifically to overcharge their senses and search, and no unexplained phenomena that are best explained by a hidden Guardian. There is absolutely no evidence for the existence of a Guardian despite a hell of a lot of very dedicated searching.
And I'll give you word-of-QM that no, there isn't one. No need to be coy about this one; they picked Garenhuld specifically because of what a backwater it was in every respect, and this is just another aspect of it. Even gods or superpowered philanthropists feel absolutely no reason to come to this place.
...actually, in the context of DBZ: before you get up to the tier where people are using god ki, is there an actual
difference between the gods and mortal, superpowered philanthropists?