Follower of the Way
I think what tends to work best is having the right balance for yourself--which will vary from one DM to another--of prep and improv. In general, I'm pretty good at making ideas fit together, even if they weren't planned to fit together originally. It's a skill I just sort of...have...without really knowing how I got it. This, plus a couple of careful tools, allows me to do relatively light world-building in advance.Their instinct to have a grasp of this stuff independent of player interest is a good one, though.
I'm not interested in engaging with the commerce and trade aspects of the game at all.
The trouble is that I want to engage in heroic labor for hire by settlements in dangerous environs.
So I'm forever talking to the beleaguered Margrave of Trollwaste March about how their existing fortifications are usually sufficient to address the monster activity thereabouts - but only just, and the activity has increased of late.
And I can't help but ask, what is the extent of those fortifications, and are the watchtowers manned, and are the men armed and armored, and have they acid flasks or alchemical fire?
And I know that every "yes" in that conversation is a chain of skilled tradesfolk and fighters and resource gatherers and all manner of people vigorously pursuing assorted calorically-demanding not-farming toils.
I would very much like for my DM to understand enough of that stuff to convey a world in which it's all sensible enough that I can relax and get on with being a heroic sorcerer-diplomat who doesn't think about commodities trading at all.
But another skill that is supremely useful: researching what you need to know. I know an awful lot of miscellaneous random facts, but much more than that, I know how to find whatever miscellaneous random facts I need for some obscure question I didn't expect to answer, most of the time anyway. With decent google-fu and a bit of practice, one can meld unexpectedly-useful ideas pretty quickly into existing established information. Then, build things up, and write things down. Notes are incredibly important and really help bring things together.
I guess, in a sense, you could argue that I did a lot of my prep work purely through just researching and getting myself in the right frame of mind. Reading relevant material, looking up historical information, reading other systems' setting info for similar settings...all that exposure is a form of prep in and of itself.
So: You don't need to have written up those skilled tradesfolk per se. But you do, as you say, need to think about why the kingdom has fortifications, and why they're insufficient now but were sufficient before. If you have a trade empire (or, as is the case with my setting, more a very loose trade league of variably-squabbling city-states), you should think about what each city offers in goods, and what goods are highly desirable. If you want one faction to have a lot of power, you should have a reason why it has a lot of power.
The smaller details, the workers, the supply lines, the barracks...those can be filled in as you go. But you need at least a bird's-eye view. That's the body. Once you have a body, you can dress it as you like. But if you try to build a body beneath clothing you've already sewn...well. You'd better hope your improv skills are up to the task.