Really like the "state your assumptions out loud" move, it's the step juniors skip most. Two things that pair well with your 30-minute flow:
- Write the numbers on the board, don't just say them. When you later choose a cache or a shard key, you can point back at "~1,200 read QPS" and the trade-off sounds reasoned instead of memorized.
- Pick your deep-dive before the interviewer does. Around minute 15, say something like "the riskiest piece here is the feed fan-out, I'd like to spend the next 10 minutes there." It shows judgment and keeps you out of a random rabbit hole.
One small extra: close with a 30-second recap of what you'd change at 10x scale, since that's the last impression the interviewer writes down.
(Disclosure: I work on Linkjob AI interview assistant, a tool for interview practice and live help.)