If you want to upgrade range, I'd actually spend my money on Azure dragons. A range increased AD would cost 90 koku less than a black bear, and in my opinion with all the upgrades, the difference in reload and accuracy isn't going to make too much of a difference especially if you put in one or two of the foreign marine units.
So a fully upgraded AD with range increase is going to potentially cost 1340 koku (or even as less as 1280 depending on the upgrades you get). For as little as around 8000~8500 koku, you can get 5 ADs with 150 range plus one foreign marine. That's a saving of 450 or more koku from what you would have spent from upgrading BBs.
In my view, the whole point of getting range upgrade is not only to shoot at your enemy first, but also to draw them closer to your shorter ranged units. So I personally wouldn't invest too much into giving all my line infantry increased range. Rather, I would put one unit with increased range at each point of my formation, so the enemy (who hopefully won't have more increased range units than me) will have to walk into the range of my less upgraded units. But that's just my view on battlefield tactics, and you guys might have a different view altogether