> I can’t think of a better way to get a sense of how a candidate reasons about things

Some ideas to help you: ask the candidate something underspecified and watch what they do first. Do they ask clarifying questions, make their assumptions explicit? After they answer ask what would change their mind, where does that break down? Pick a topic they know and ask them to explain it to a smart non-engineer. Make them estimate something they can’t look up (forces them to decompose, bound, and calibrate). Once they’ve proposed a solution to a question, change the constraints to see if they can adapt or whether they’re stuck.

What you want to evaluate is dynamic reasoning, adaptability.