Current Job Vs. Side-Projects

It depends on what your goals are with your time.

• If your goal is to get the most enjoyment building software and you're not getting much enjoyment from work, do just enough to consistently meet expectations from work and focus your spare time on side projects.
• If your goal is to maximize your income, learning new things with side project won't provide that much value (unless you have an idea that has a clear path to monetization). Learning new tech can give you a better perspective on software and give you some nice hits of satisfaction when you've built up decent mastery over new tools & libraries, but often times it won't carry over to your day to day job (since the main bottleneck is convincing others the investment is worth the time).

If you want a midway-ish balance, you should look to identify bottlenecks in your day-to-day work and look to learn technologies that might mitigate them. Especially if your coworkers are experiencing similar issues, you'll have a stronger case to explore certain tools and technologies.

I don't know about your characterization, both Alex and Rahul spent lots of time building side-projects and they both had top-level income. And I think they both credit side-projects with making them great engineers. Wouldn't that indicate it's always important to be working on side-projects, or are they just anomalies?

I'd say it's a mix of them being able to sustain their current level and finding enjoyment in their side projects (Alex also explicitly took a lower level at Robinhood so that he could have more time to take things on outside of work). My post really isn't about characterization fixed paths, but about identifying goals. If you do not have a clear goal in mind for taking on side projects (besides just wanting to do them), then you'll likely won't make signficant progress on your side projects and won't really have that much to take away from the experience.