TL;DR: as a dev, what's the most valuable skill to learn when wanting to become a more well-rounded solo founder and how do you learn that skill the quickest?
Some thoughts (I could be entirely off-base, hence the TL;DR):
I have a side project in mind. I know nothing about marketing other than some basic ideas (I think I read half of the book Traction at one point?).
I was wondering, should I:
1. Learn marketing
2. Start a side project?
Goal: in 5 years from now, I want to be well positioned to launch a startup? What startup? I'm an idea guy by heart. I studied a master CS 5 years ago + did a software engineer career, in order to implement my ideas. In other words: I'll think of something that I find fun and will solve a problem.
Also, if learning marketing first is the smarter option. Should I learn marketing by taking a course, or should I take a side step (or step back) in my career and do a marketing role?
Try to lose this notion.
The key to success in a startup (apart from timing, luck, having the right idea, grit, focus, etc) is determination to see things through to completion. You have to believe in your idea and see it through to the end, where 'the end' is hopefully an exit of some sort probably years in the future. If you see yourself as "an idea guy" you will be distracted by the new shiny thing you think of, and if that happens when your startup is in a slump you'll give up before you see success.
You do need to be "an idea guy" to build a startup, but all the ideas need to be about driving the core idea forwards. You need to see yourself less as "an idea guy" and more as "a <whatever the startup is about> guy".
To that end, I'd recommend starting a side project to see if you can grow it without getting bored, giving up, etc.