I watched all but one of Lamport's videos on formal specification with TLA+, though I yet have to tackle some project with it. Right now I got my hands on Tufte's "The Visual Display of Quantitative Information" as per some HNer recommendation (thank you!). Yeah, I lose interest quickly, eh. There's so much cool things to learn that in the end I learn nothing well. Bummer. edit: grammar, spelling
If you consider how knowledge decays, your strategy makes more sense. You get to something like Pareto proficiency in a subject and can then ‘reactivate’ that knowledge more easily in the future, should it be required.
You are also of course aware of that knowledge in the first place, which may be even more important. In the timeless Rumsfeldian: you have pushed back a bit at the unknown unknowns.
The alternative path of deeply learning something you may never apply (e.g. what happens to many PhDs) seems inherently less desirable, in my opinion.