Perhaps rather than have the smoke from fires always visible, have the game treat it like it treats human/animal tracks/presence; that is, a *chance*
*that the character sees/hears it unprompted;
*of the relevant icon being visible on the overland map (and when getting [l]ooked at, a relevant description popping up);
*that the character sees/hears it when "prompted" (that is, actively looking for such things through tracking),
with written descriptions hinting at the likely size of the fire and the overland map icon differing depending on the size of the fire?
E.g.
"As roaming ahead[,] something suddenly captures your attention.
You notice here/[to the -direction-] a thin wisp[/small spire/thick column/heavy cloud] of smoke." ([,] because the current used statement doesn't actually have a comma, but ought to)
and a similar description when using tracking on the overland map, like
"Using: TRACKING
You walk around a little looking for tracks...
You notice here a [thin wisp/small spire/thick column/heavy cloud] of smoke."
and the description when [l]ooking at a fire overland-map icon, something like
"You see [a little/some/a lot of] smoke in the distance there."
As for smoke attracting other humans, perhaps a radius surrounding the relevant overland-tile depending on the fire's size in which already-generated human NPCs have a heightened chance of being attracted? In other words, the smoke not actually *generating* NPCs to be attracted, but if already-generated human NPCs either already are in the relevant radius when you start the fire or pass through said radius while the fire is burning, there is an increased chance they'll head to your campsite?
Perhaps one overland-tile in all directions surrounding the fire-tile for smaller fires (fires lasting anywhere up to a couple of hours), three for medium fires (lasting less than a day) and five for huge fires. (lasting more than a day)
Many, many (smaller) fires should probably be handled differently--like torching a raided village; torching a forest to make room for farmland; anything resembling a wildfire--fires and smoke attract humans, yes, but if it looks like there's a rampant wildfire most folks will opt to head in the opposite direction.
In any case, those are details that would probably require testing by Sami to figure out what works.
Behaviour should probably also be dependant on the kind of NPC they are--townsfolk shouldn't all start heading over en masse to your camp two tiles away just because you're torching the stack of poor boards they weren't interested in. On the other hand, lost woodsmen looking for help would be rather strongly attracted to a campfire--perhaps even moreso than the hostiles.