Ask HN: Most effective path to become a solid systems practitioner?

Context: I'm a mid-tier dev at a BigCo, and I was never classically trained as a computer scientist. Lately, I've been interested in problem spaces where having a deeper systems knowledge (think databases, streaming systems, operating systems, distributed systems etc.) is incredibly beneficial. I haven't found a straight forward path to learn all this short of maybe doing a Masters degree.

Is there a set/sequence of resources/MOOCs that HN could recommendation on getting ramped up on these topics. I understand that it will likely be a multi-month journey at the very least, but I'm tired of hitting a ceiling on what I'm able to contribute to due to my lack of knowledge.