So at first we're introduced to Kami as "God" but then....

[Herms]

I think we can blame the confusion on the language barrier, at least a little.
Japanese doesn't use articles like "a" and "the" the way English does. It has demonstratives and such ("this" "that" and "that other one"). But it seems like the lowest level of god in DB is planetary, so "kono kami" / "this god" just wouldn't come up in conversation.

Yeah, in Japanese there's no real distinction between "a god", "the god", "God", etc.

Of course, it's also simply not true that specific gods in belief systems with multiple gods are never called simply "God", but that's probably going beyond the scope of this thread.

Dabura had been to Earth 300 or 500 years ago?

He says he "had Earth checked out" about 300 years ago, implying that he himself didn't go.

[Piccolo Daimao]

I call the God of Earth "God" for short, so I don't see what the problem is, as long as people know that it's a polytheistic set-up.