I ran a very similar experiment. I don't recall the exact dates, but something like 2016 - 2018, and left Wealthfront as a result. I was under the threshold that incurs management fees, but Wealthfront was outperformed by S&P, at least over my time frame.

I never held anything with them during a market downturn, so I do wonder what that might look like. Potentially the lower returns would be justified by the existence of a hedge or holdings in lower-risk assets.