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.