I'll add a trick that I think helps: work for very small companies or ones with very small software groups. A startup qualifies of course but there are lots of small companies where you can work on interesting stuff without the risk of a startup.

Large companies tend to have enough employees that people tend to specialize - it's an optimal arrangement for them[1]. Your "reward" for becoming the expert with technology "X" is that after a while you mostly get assigned to work with "X".

At a small company, that is less likely to happen. At my last job as an employee in a 2 developer company I would find myself doing high level GUI (WPF/C#), low level computer vision (algorithms/C++) and bit-banging stuff out a communications port while watching the responses from the laser controller using a data logger - all in the same week. A small company can't afford to have a developer dedicated to a single thing so by necessity you wind up wearing a lot of hats and learning a lot in the process.

[1] See theory of comparative advantage https://en.wikipedia.org/wiki/Comparative_advantage