上次 means “last time”, as in the previous time or last time something was done
下次 means “next time”, as in the next time or subsequent time something will be done
Now, “last time” and “next time” sound great with each other, but the former can be a little confusing on its own. When I hear “last time”, I think of finality, like “this is the last time I’ll say it!”
I think by changing the primary definition to “previous time” the meaning will be a lot clearer, then just add “last time” as a synonym.
I’d agree with having it on as an alternative meaning! Sometimes studying on HH I feel like I’m relearning English. Like, I came across the word for “persecute” and realized I never actually knew what its concrete definition was. Or, the word for assemble (ji2he2), I realized it meant “to assemble a group of people” rather than like “to assemble furniture”.
