That’s kind of what stationary mobility is for. Of course it works differently, and you have overlap limitations, but as long as most of your scene is static, performance is quite reasonable. The shadowing of dynamic objects is what can be quite expensive (which can be disabled if the scene allows it).
I’m not sure what’s the issue you have with increasing density. A density of 50 is usually good enough for interiors with static lighting. Static lighting + capsule shadows if properly configured can be quite good (there are some useful cvars to change).