[nvarsj]

Most people can't casually throw away 10k to get better at interviewing...

I'd say for us non-wealthy mortals, the best approach is to find another engineer who will conduct mocks for you. Short of that, maybe do one or two paid mocks to calibrate yourself, then practice on your own.

The sad fact is the "system design interview" is not really like actual system design at all. It's more about following a specific format that everyone expects and knowing the ideal solution up front.