Is it possible to apply dB/volume changes in the timeline rather to an actual audio clip?

Basically, no, you can't control volume on a whole Storyline, or Lane of clips.

The only solution is to pack them all away in a Compound Clip and adjust the volume of that CC.

But then you can't work with edits and sound at the same time.

We've been waiting for Roles based mixing and mastering tools for over ten years now.

For changing the music, you could try putting the music inside a compound clip, and adjusting the audio on the compound; then you could edit the compound to slide or replace the music, but when working inside the compound there is no clear way to identify what part of the video it corresponds to.

For being able adapt to adjustments in the dialog, you can drag a rectangle to select multiple keyframes and move them all at once.

If you need to replace a piece of audio you can copy it, replace, and then paste attributes to put the audio keyframes back into the new clip.

Is it possible to fix or apply dB adjustments to the timeline rather than an audio clip, so that dB/volume changes stay relative to the time regardless of which audio clip you use underneath?

I don't understand what you mean by applying volume adjustment to the timeline. Volume adjustment takes effect after you apply it to a clip.😓 Please show me the screenshot and explain specifically which part is bad and how.