How do you balance building new features versus working customer acquisition?

This is a very good question. But the answer depends on so many things. It very much depends on the stage / maturity of the product, on how much cash you have (you could just pay Google to do all the customer acquisition work, for example). It depends on what you are building and on your market. It also depends on your job, what is it you do in your business (in my opinion, "founder" is not a job). If you are a CEO, you are defining strategy, building team and managing your business. If you are CTO, you are a lot about evangelising and building your product and technology. If you are a CMO, well, you are 95% about customer acquisition. And so on. From a very high level, getting more customers and building a valuable product are the top 2 priorities for most businesses. You can go 50/50 on features vs customer acquisition BUT spend 110% of your time doing it. Sorry, I can not be very specific. It would be wrong to suggest anything with certainty without understanding your business. ...except for, perhaps, founders really have to put 110% of their time into it.

It is way easier to acquire customers first and then build features than the other way round. I have seen a lot of product startups share a Trello board of their product roadmap with their early customers.

I'd take whatever your unique value prop is and focus on improving it through customer feedback. Increasing the number of features isn't a good way to increase growth early on. It'll just lead to more issues and complexity.