This might not be responsive because spare time isn't something I can count on filling with other than sleep. If you don't mind a more general response, what I do is keep a list and when the next paying project comes along I'll try to use something off the list to realize the product. As a ferinstance, right now I have Elixir, Rust and FreeRTOS on the list. Two days ago a project came down from on high where even though I can't demonstrate that I need an o/s (it's not internetworked) I'll implement over freertos and get that off of my list.