You don't need to entirely forget this. I've made a habit of regularly seeking out job opportunities and interviewing even when I'm entirely happy with my job, which is to say I've done a ton of these kinds of interviews (on both sides of the table).

Unless the initial question requirements are insane (build Twitter at Twitter scale), I start with the smallest, dumbest thing that will work. Usually that's a single machine/VM talking to a database (or even just SQLite!). Compute and storage are so fast these days that you could comfortably run your fledgling service on a Raspberry Pi, even serving three or four-digit QPS depending on the workload.

Of course, you still have to "play the game" in the interview, so make sure to be clear about how you'd change this as requirements changed (higher QPS, more features, etc)