I found the best resource for myself is Anders Ericsson's The Road To Excellence. The biggest problem I keep finding throughout my own career is I keep losing sight of where I need to focus to push my game forwards. Most of the online resources are aimed at the overcrowded beginner and mid-level end of software development, so after a certain point, you're writing your own syllabus to advance. Or you can just go broad, which I think is pointless. Ericsson highlights deliberate practice, the idea of finding your discomfort zone, where it most matters. I think there is a lot more to it than he writes in the book, but it's a good introduction on the topic.

The thing about ageism is just a simple matter of interests. It's reasonable to think managers and teams are looking for the best talent, but it's more like the game show Survivor in that people are looking for talent only up to a point, people need to feel your talent isn't going to expose their weaknesses. So as you get better past a certain point, it really gets worse for you trying to break into new groups.