Feature suggest: Post plugin/channel strip and pre-fader gain plugin or automation.
It is a sub-level for the after-plugin loudness automation adjustment, then the volume fader to balance between the different parts.
Otherwise, it needs a group bus to balance between parts.
Just pick up the free bundle (for the MUtility module) from MeldaProduction or the free Gain Suite from Blue Cat Audio. This will do just what you are asking for.
Thank you. I know these. But I want this feature to be integrated within Cubase.