If you get into that situation then it's probably because that API is critical and irreplaceable (otherwise you wouldn't be tolerating its problems), so you really don't want to get stuck and be unable to query it. And if you can tolerate a SPOF then there's no reason to bring Redis/Postgres into the picture, you might as well just have a single server doing it.

Plus it's just good practice that I'd want to be following anyway. Once you get in the habit of doing it it doesn't really cost much to design the dataflow right up-front, and it can save you from getting trapped down the line when it's much harder to fix things. Especially for an interview-type situation, why not design it right?