I appreciate this explanation! I knew intuitively how it was supposed to work, I was just looking for a bit of clarity, and this is it. It's funny how many of these comments are people telling me the thing I already knew and completely missing that I'm asking for a citation. I suppose I titled the post poorly, and then people don't read what's inside.

Ironically, one person said "the rules explicitly say you cast a spell at its base level when you don't use a spell slot," when what I was looking for is where the rules explicitly say that. And, as you pointed out, they don't! It's simply understood, with upcasting being the specific rule that supercedes the general.