The biggest red flag to me seems this: "second founder ducked out after about 8 months—with a sizable percentage of the company." The founders should have been on a vesting schedule to prevent this -- or every founder has an incentive to just duck out and keep all their equity. IIRC YC requires such vesting agreements, and for good reasons.

Really depends on what size able means. Let's say the founders split the company 50/50. They're on a standard vesting schedule. One of them leaves after 1 year. They will have 12.5% of the company.

Do you consider this sizeable? I certainly would, and this will happen even if you have all the standard mechanisms in place!

There's an argument I've seen that founders should be on a thoroughly nonstandard vesting schedule, for exactly this reason. Slower vesting, a longer cliff, an event-based cliff, or some combination thereof.

It's a decent assumption that all successful, unpurchased startups take at least 3 years to hit it big. Given that, you could reasonably put a founder cliff at 2 years (with early vesting on buyout) as a show of faith.

Of course, the other argument is that if a founder leaves with 12.5% of your company something else is terribly wrong regardless.

My first real job was at a startup that was at one time a one man show (though it grew and shrank over time). The founder was an ego maniac and a micro-manager who had big visions of financial success but no clue on how to achieve it. I heard a lot of stories of an endless parade of previous employees (which should have been a major red flag) and while I was there for only about 2 years the company cycled through people.

That's the case coming from my current job. Starting a new job on Monday. He had his fingers in everything, launched a product that was not ready for public release. Constantly micromanaged me out of using standard SE and EE practices despite my advisement against it and my push back. Suffered the consequences, manufacturers wary to sell because of all the problems with it. Can no longer support my employment because there have been no sales.

Have any tips for us solo, first time founders, besides "no needless meddling"?

In our case, the goalposts for what the product should be kept moving every 3 months, which was probably the largest single problem, it caused most of the friction imo. It didn't help that we were a hardware startup, so changes couldn't be made as easily. A solution to that would be to agree upon (and document) what the MVP is, and not change course until you've validated that the idea doesn't meet the market's needs. We had a single client/partner, their desires ended up driving a lot of changes late in development that ended up being incompatible with earlier engineering decisions, but the founder pushed them through. We ended up not being able to do environmental testing because of the time those changes took, and the first time it rained the cases for our electronics filled with water.

I'm also now a solo-ish founder and I only have technical/team management experience, so I'm learning too :)

This happens in non-solo founder companies too. In fact, I would expect it to happen slightly more often in companies with multiple founders, because they jockey over what the roadmap should be. When there's a lack of leadership at the company it's a big problem, but it happens at all types of companies, not just small solo founder startups. I think you are conflating the problem with the fact that there is only one founder.

Who puts you in line when you're in the wrong? Do you have a CXO who feels truly comfortable telling you something you don't want to hear, even if she knows she can be fired on your whim? Do you go to lengths to make it be known that you welcome all critical feedback?

make sure you have someone close by who feels comfortable telling you when you are wrong

Also, don't found companies with friends (unless you've successfully worked with them before), because people tend to let things slide with friends longer than they would with an average co-worker.

Know how to leadership.

Create a safe space for quiet negative feedback or misgivings. Avoid shooting messengers.

Delegate, then trust the people you've delegated to.

Have a plan, then stick to it. Changes in the plan must be communicated.