My team and I built this. We tried to make it work on smartwatches, but necessary Hz, battery life, and UX limitations (ie: being in a "fitness session" to access necessary sensors) all proved to be an inadequate user experience. So we just created dedicated hardware and firmware.
This essay points out valid weaknesses in the lean startup approach, but really loses it's edge when it suggests that "social proofs" - such as the ability to hire highly paid talent and/or acquire funding - are better, higher priority signals of validation. Yes, customer interviews too early on a "guess" that is barely thought through is not an efficient use of resources. But trying to hire and raise is a worse alternatives.
The lonely aspect is incredibly difficult, especially if you're an extroverted person that draws their energy from collaborating with others. I also find the lack of accountability to others as hard. I know there are services that cater to this type of founder/freelancer.
Check us out here: https://immutouch.com/
Here's TechCrunch's write up on us: https://techcrunch.com/2020/03/09/dont-immutouch/