Indeed, such technical vocabulary is very hard to translate. In English, we have had several generations of attempts, and it is still very unsatisfactory. Still, I would encourage you to persevere with the attempt to find a native idiom, rather than falling back on imported Pali terms, or worse, importing the English.
You can see a previous discussion of this point here:
I ended up keeping “aggregates”!
The basic idea of khandha is a body, aggregate, trunk, bundle, or mass. Each of the five khandhas is a “mass” of something (form, feeling, etc.) in the sense of a bundle of all things of that type.
Making it harder, the idea connects with a range of imagery around “fire” that is hard to translate; think of a “bundle” of firewood.
I agree, the fact that the Buddha just mentioned them in passing implies that it was an already-known technical term.
No, that doesn’t sound right. It needs to be something like “category” or “mass” or “heap” or “collection”. In the phrase dukkhakkhandha we can use “mass of suffering” quite nicely. “Bundle” is nice because it implies that the things are “tied together”.