I like the idea of restricting a settlement to a biome rather than a radius. However, as you said, it might cause some problems. A possible solution is to allow certain buildings (mines, roads, 2x2 towers eventually) and activities (gather pigs, cut trees...) to be built in adjacent biomes, letting citizens gather ressources from adjacent biomes, but not live there.
I'm leaning towards that approach. Depending on the size of the regions.
would open the possibility of peaceful interactions down the line. And/or conflict between masklings.
I think, once I generate multiple Maskling tribes, that conflict between them will happen automatically. Peaceful relations will take some work, but will really expand the possibilities of Bronze Age.