Playing with Shadow Bias parameter can help with banding issue. And if you want more indirect lighting i suggest using a white material for surfaces getting directly lit while baking and change it back to their own materials afterwards.You can also increase Global Illumination Intensity in post process volume after building the lights. And since you said theatre i’m guessing the seats in the back and middle end up too dark, so make sure you have reflection capture actors as they help capture the light from lit areas. If that’s not enough you can also experiment with capturing a cubemap of the theatre and using that render target for post process ambient lighting.
If you still have issues feel free to post some screenshots.