If a stationary light Indirect Light
Intensity is set to 0, it’s the same
as a dynamic light?
It would act like one but it doesn’t automatically make it a dynamic light.
Post Process:
This is just something you’ll have to design around to get what you want or make some choices with how you want lighting to be handled and controlled.
As a final suggestion, you could simply make the dynamic and then use a non-shadow casting low-intensity light or lights placed around the room to fake bounce light where need when it’s lit and disable all of those when you turn the main source light off.
You may also want to play around with the Min Roughness setting depending on reflective surfaces, the option for Use Inverse Squared and Light Falloff Exponent to get some more consistent results as well. It becomes art tweaks really at this point with what you want it to look like.