Past a certain point the +1 isn't something you can do on your own time. If you're looking to shift into more of a management role that's obviously hard to learn on a side project unless that side project is a full company with employees. Even pure technical skills can require certain scale; you aren't going to learn how to use k8s appropriately on the side because whatever you're doing on the side almost certainly shouldn't be using k8s at all.

"Keep doing what you're doing but half the hours and double the salary" is something I would be tempted by, but it would be with the idea that I'm focusing on something other than developing my career.