In your experience, is it possible to negotiate work at a rate where one would only be working 2d / wk? I'd imagine it's hard to negotiate such an agreement with a company because they want work done on a faster timeline?
But there's a lot of work that isn't your hourly billable that goes into running such a business too, administrative overhead, client relationship management/development, etc.
I personally ultimately decided this kind of consultancy was not work I found rewarding. I also was not one of the owners/principals, and was not there very long, so I'm not really an expert in this kind of business, and probably there are people who might be better to give you advice. But since you asked.
Sometimes a company really does need something yesterday and you're the bottleneck in getting something released/getting revenue/etc. Bit I've also taken on writing and video projects for example that were components of a marketing campaign that was scheduled to launch some months down the road so it was a pretty part-time thing. Mostly I've worked on internal projects but the same principle applies.
With new feature development it would be harder. But you could negotiate a weekly fee based on a 4 day work week, and then get so good and efficient that you do the work in 3 days.