Coming from another experienced dev who was frustrated (early) - one tip. Don't over think it. I was tripping up in two system design interviews that ultimately boiled down to them wanting laughably basic answers (one pagination, the other a db transaction). Don't be afraid to deal in basics and then directly ask if they'd like something more detailed, or not. Many places, especially non-elite ones, treat system design as a _very_ basic test of knowing how pieces fit together.