There are definitely small (<10 people) companies doing millions with ecommerce. My inclination is that most of them are doing some/all via Amazon these days. But even outside of Amazon yes, there are some people doing that. Headcount is less important than technical headcount - there may be 8 people in the company, but they just outsourced a woocommerce shop and market the heck out of it.

Margins matter too, not just headcount. Someone could do $2m/year but make less than someone doing $800k/year with better margins.

Doing perf/security reviews of a niche like that (woo, magento, etc) would probably still be a place to focus and make money, if you want to stay 'back end' (or at least technical). But even then, some of the measurable stuff will be needed - "we reduced the load times by 45% on the checkout page, and conversions went up 17%, adding $87k in profit last year".