My suggestion is simple. It could be implemented as part of the original Auto Duck as a box to be checked (in settings or Auto Duck menu) or a separate Effect altogether.
The Custom Auto Duck would be an Auto Duck that is independent from a control track, so you don’t have to add a sine wave track in order to duck your music. Other than that, it would be just like a normal Auto Duck.
It’s not entirely clear to me what the end result you’re trying to achieve is. Do you want to change the volume throughout a song? Then the Envelope tool may be what you’re looking for. Or do you want to compress the audio over an entire track? Then a compressor would be the tool of choice.
Maybe you’re looking for an envelope-follower plugin, (which is the inverse of autoduck).
My suggestion is easy. It could be implemented as part of the original Auto Duck as a box to be checked or a separate Effect altogether.
The sine-wave-track is a workaround to make Audacity’s Native AutoDuck into an envelope-follower.
e.g. AutoDuck the constant-amplitude sine-wave-track, then use that track as a control-track to Autoduck other tracks, which will then have the same envelope as the original audio.
The simpler way is to use an envelope-follower, e.g as provided by Steve’s Dynamic-Mirror plugin.