Really, I think the target should be viability for a variety of play styles. So, there should be a capstone archery weapon like the greatbow and the goal should be for it to be powerful and fun to play with, and for the steps along the way to be powerful and fun. Likewise there should be good pistols and rifles, with powerful endgame versions of those, same for unarmed and armed combat styles, et cetera.
With that in mind, I’d say bows and crossbows should be balanced such that the capstone weapons, be they greatbows or some fancy compound bows, are difficult to obtain and loosely comparable to other capstone weapons.
I’d say needing 18 strength is a good start on “difficult to obtain”. Yeah, fine, you can start with that, but only if you’re building your character around it, and I think it’s debatable how worthwhile that is… Certainly it still counts as unusual. You could also add in the requirement that it be made from more complex materials than simply wood and string. I’d personally suggest working in a multiple stage crafting process, where you build a bow blank and treat it with glue, then have to let it cure for a day (using the same code as fermentation) before using other hand tools to go to the next step. Making a longbow is supposed to be a big involved process and currently it’s a matter of a two-by-four, some string, and an hour of your time at a low fabrication level. A lot of the problem starts there.
If it’s made sufficiently difficult to get a greatbow and wield it, then its damage really isn’t all that out of line. It’s more powerful than high end pistols and less powerful than high end rifles. I’d say the balance should shift a bit, hugh calibre pistols should probably outclass high end bows, but they’re still operating on the same level. As long as the bow isn’t really something one can just build on day one, it’s a pretty reasonable weapon.