I started university as EE, and wound up graduating with Computer Engineering (basically 50/50 EE and CS). 95% of my professional work has been nothing but software, but I've got some (laughably simple) professional hardware experience.

I literally can't keep my head straight without sketching some schematics. It's also super helpful when I'm prototyping and tear down a component to build something else (say, to reuse a microprocessor or expensive component like a GSM module) but then I want to go back to that first setup I had previously.

Try asking someone who really knows what they're doing for some non-trivial help and show them your rat's nest and no diagram and watch smoke pour from their ears as they try and remain calm and helpful.

I think you're on to a good idea though, the UML you practiced provided value (likely not because we all love UML) but it forced you to meditate (for lack of a better word) on design choices and decisions. You began iterating in your head and built some good reflexes. You were practicing your craft in a more robust fashion. I can't imagine spending time with schematics won't be helpful.