As it was explained to me, dhatu means something like "aspect" or "tier". Meaning, it's a cross-cutting principle found in the whole that's delineated when you analytically disect the whole in a particular way.

So, when you analyze experience, you may delineate "the visual aspect" - present across many rather different experiences. When you analyze external dharmas, you may delineate "the aspect of solidity" present across many very different objects etc.

Today we could delineate e.g. the electromagnetic aspect, the chemical aspect, the  informational aspect. All are elements or principles of the whole existing on different "analytical planes".