There are a million things to learn, how do you pick which to learn first?

I've been developing an app for the past year and there are so many things that I feel that I still want to learn and not nearly enough time in the day to learn them all.

For example, at the front end of a business we have marketing, every startup founder needs to invest time in talking with potential customers before they build something that no one wants. But I also will need skills to develop that product, should I focus on designing a really great mockup for a better user interface, clean up my current project's code to be more maintainable, work on how to port my web app to a mobile app, or take a course on how to add that new AI feature that I've always wanted to develop.

I think that the obvious solution is a just-in-time mindset (learn what you need when you encounter the problem), but that runs into the issue of "EVERYTHING IS A PROBLEM RIGHT NOW!". If I was giving advice to someone, I know the importance of talking to customers and would make sure that's involved, but coding skills could help land a software engineering job that a user research commitment couldn't. And if you can't build an excellent product, then all the advice in the world can't save you.

I suppose it comes down to invest 50% of the time in both of them. You need them both after all product and marketing. But I'm curious how you guys balance all of the things that you could be learning/working on.