Note, CLIP_PT_track looks a bit mess UI wise, improvements here are possible. Also I am unsure how useful the subpanels are here.

I am not really sure this improves usability.

@Sebastian Koenig (sebastian_k), mind having a look here? :)

The goal here is to reserve the toolbar just for tools

As long as we don't actually have a proper tool system, we should stick to the current solution. Sure, it deviates from the rest of Blender, but simply putting everything in the sidebar is no solution either.
Yes, the Clip Editor UI needs to be updated to be consistent with the reset of Blender, but then in a proper way with an actual tool system. That might also then benefit the Mask Editor.

I agree 100%. Are there mockups of the new design and workflow somewhere? What the tools are, how they work, why they were chosen over over other approaches, and how this is better than the current workflows etc.

There was some design in T83612 maybe more discussion needs to happen there first, My first plan in moving to active tools is to make the toolbar consist of only operators, this means moving properties to the sidebar.

Off-topic but please provide an image of before and after changes when submitting UI diffs. D9683 is a good example. Makes reviewing faster without having to compile if the changes are purely aesthetic.

Having agreement in the design before starting code kind of comes without saying ;)

To avoid temporary usability regressions in official Blender it will be better to work in a branch: do required refactor first, and then implement awesome new features. This way you will also have way more freedom for doing quick iterations over tools development to collect users feedback.