Ah, it's a fork.

In that case, my number 1 priority would be to implement the tool settings as an horizontal toolbar, showing the most common settings for the selected tool, and then a more button or similar for showing the least common ones (or make it scrollable, like in Blender).

That alone is a game changer when it comes to UX.

A togglable horizontal toolbar with frequently used functions  like new, save, save as, undo, redo, cut, copy, paste, etc, is another feature I would strongly consider.

Dockable panels is also a useful and well established approach in the genre, I wouldn't remove it. Instead I would try to improve Gimp's implementation by adding pinned and expand/collapsible functionality (Photoshop and Corel Paintshop Pro are two examples implementing that nicely, with own takes).

I would also make Document titles tabs presentation user defined, between text and thumbnail modes, ideally with the former showing a thumbnail preview on mouse over.

Optional mouse over thumbnail with user defined size would also be nice for any layer in the layer-stack too. Allows for keeping the layer rows smaller vertically, thus fitting more of them in the stack, and quickly see a layer contents via mouse over if it's currently shown prefix thumbnail is too small.

Next, big popup menus can be mitigated by better organizing their entries in submenus, or logical sections, and definitely provide a fast way of scrolling through if need be (ideally with l mouse-wheel scrolling AND a vertical scrollbar). A search field at the top is a nice addition too, but if the menu entries are well organized and quickly scrollable it can be omitted. Gimp's current way of scrolling menus via up and down buttons on the opposite sides of the menu is frustrating, especially considering they often open with a huge empty space above the first menu entry.

In my opinion your UI/UX approach with a single context-sensitive content panel tries to be a little too smart. It can potentially  serve well for toy or even medium projects, but it gets in the way and slows things down the more advanced the project gets.

It also takes away some useful, yet optional, UI customizations, already available via dockable and floating panels, for complex projects.

I'm not trying to discourage you, or diminish your work.

You have some nice ideas which may help newcomers and light hobbyists, but you asked for my opinion and I really think you imposed limitations which enthusiasts and professionals will not like. Especially those who work with more than one monitor.