Thanks for the reply 
I did try quite a few different approaches, although I can’t remember if I tried exactly what you’re suggesting. I’ll give it a whirl when I get a bit of free time. For now, I got it working well enough using the method I mentioned - reducing nav generation radius when NPCs are far from the player, and only increasing it once the player gets close enough for it to matter. I think it was just a matter of playing with the values to find that sweet spot between what you want in an ideal world and what’s acceptable, performance-wise.