I'm not envisioning the OP as working through musical concepts on a sheet of staff paper with a pencil.
Fair enough but neither am I. It's not my intention to send the OP down the rabbit hole or anything. Neither am I suggesting that they abandon physical practice on the bass. On the contrary I believe that with some additional theoretical knowledge under their belt, the OP should shed as much as possible.
However, the OP is saying that they are comfortable enough with their proficiency on the instrument, they now want to learn some accompanying theory. Additionally they say that they want to make a transition from player to musician, which I think is the most important distinction.
For myself, I concluded the difference between the two is that a player is proficient in their instrument, while a musician has acquired theoretical concepts which apply to all instruments. I personally interpret musician to mean: somebody aware of music as a whole topic, not simply within the framework of their own instrument. This is so they can deal in concrete terms with themselves or other musicians playing other instruments. Especially the latter, if we all use the same vocabulary to communicate musical concepts, there is a greater potential for the exchange of information.
So it might be okay for them to put down the bass and pick up a book for a while? And split their time of course between research (mental practice) and actual physical practice.
I'm not saying become a professor of bonkers useless quantum-meclassical knowledge. I'm simply saying that basic theory is universal across all instruments and more useful in becoming a well rounded musician, who can converse comfortably on mainstream musical topics with another musician playing any instrument without feeling lost. Not simply a bass player. At least more useful than experience on just the bass.
For example, my knowledge of harmony applies when I play, by myself or with others, the bass, guitar, keyboard (badly, but that's a technique/time issue), and drums. Moreover my knowledge of theory applies to any sound which I can make! I hypothesise that experience limited to the bass probably won't stop this kind of development, but it might happen slower. And that it is the kind of development to which the OP refers.
Edit: I tune drums on the kit in descending 4th's or 5th's relative to the predominant key of the material. So harmony really does apply to the drums! lol
I really see acquiring a theoretical concept in the abstract then applying it to your instrument in a practical fashion as adding an unnecessary, inefficient step.
On face value I'm inclined to agree. It does seem counter-intuitive, kind of like putting your sword away when learning how to fight, right? But the angle I'm viewing this from leads me to believe that it is simultaneously an efficient and necessary step for musical growth. All you have to lose is time. And it's not like I'm saying that they should do something harmful, I am encouraging them to expand and learn more about music. Is that of itself wrong?
Idk, those are my thoughts on it so far...