Development version: GIMP 2.99.8 Released
gimp.org
gimp.org
A general overhaul sounds too inspecific.
So, I'll open up whatever gimp-2.8 that's installed on my Ubuntu LTS laptop and take some quick notes:
* Gimp opens and an empty, arbitrarily sized window with a menu bar appears. Okay. Either I create a new image or open an existing image. The empty window resizes to fit the image. Good!
* The toolbox window is still missing. I can't draw anything. Dig around in the menu to find it. Is it in dockable dialogs, like the layers window? No, oh, it's right underneath. Open the toolbox.
* I pick the pencil or the airbrush and scribble in the window. The brush is the wrong size. How do I change it?
* "Tool options" versus "Brushes"? Over the years I've grown to dread and loath this "Brushes" dialog. It's full of all kinda of ridiculous crap that it won't even let you delete, and yet for years I, probably doing it wrong, would begrudgingly open this thing, create a new custom brush just so I could change the size and hardness, maybe accidentally click the builtin brushes that it won't let me edit until I find mine again, and after cursing at the janky behavior of the sliders give up and type numeric values in. But if I ever want to stamp an image of a green pepper all over the canvas, Gimp has me covered.
* Should I be using Tool Options instead? Probably! But there's not even a shortcut to open it.
* Oh, maybe I shouldn't be drawing directly on the image, but create a new layer instead. Where's the layers dialog? Every single thing I do in this program involves multiple layers, but none of these dialogs every re-open when I start the program, and I have to dive the menus again. Why?
* Really, why can't this thing remember my window layout when I re-open it?
* Okay, now what happened? I'm trying to draw and it isn't drawing. Oh, the layer selection region thing got messed up. How do I fix this again? Select all? Fail around in the menus a little longer, alternating between Select None, Select All, scribble on the canvas until it works or I realize I'm actually trying to draw on a hidden layer or whatever the problem is. Sigh.
* I'm ready to share my masterpiece. Can I save it? No, I have to export it. I get the technical reasons for that, and that change was made 10+ years ago, and it still never ceases to annoy me.
So, sorry, but this is approximately the experience I relive every time I use the program.
Credit where due though, I explored Krita as an alternative exactly once, and it did such a comically facepalm "Linux on the desktop" thing that I went right back to the Gimp. I open it for the very first time, I'm trying to draw with a mouse, and the cursor is invisible inside the canvas area, so I can not even see where I'm pointing. I had to Google how to fix this in the preference. Microsoft couldn't screw up this badly (sorry, Krita developers).
In case you think I'm impossible to please, Procreate on the iPad (with the Apple Pencil) is an absolute joy to use, and my favorite graphics UIs to use were a related pair of programs, "Cyber Paint" and "Autodesk Animator", which always ran in full screen, hid/unhid the whole UI with the right mouse button, and labeled all the tools and options clearly in text instead of a hundred tiny inscrutable icons.
Sorry. Maybe I'll build the newest Gimp version and take it for a spin as penance.
Edit: formatting.
So basically the vast majority of your problems with GIMP are self-inflicted, there's a local bug that only happens for you, and then there's the save/export separation that is, let's say, debatable.
Nevertheless, thank you for going into details.
As of GIMP 2.99.8 the brushes window is still the same. I think all that is due for a redesign, and the menu bar should probably be gotten rid of completely. But someone who is a skilled designer will have to contribute in order to figure out a replacement. The mockups I put elsewhere in the comments here might be a place for someone to start.
Also the name Gimp is really really terrible.
They also made some mockups of what a redesign might look like, but as you might imagine it would also be quite a lot of work to implement it: https://github.com/glimpse-editor/glimpse-nx-design
One problem with those designs is that cramming the entire GIMP's menu into the hamburger menu on top right is likely to cause more problems than it solves.
This stuff sort of works in applications with a limited feature set, such as Akira UX. I don't think it can work well for GIMP.
It's possible to find a hybrid approach, but so far, I've only seen Siril doing that, and I'm not convinced.
The designs are unfinished, but to me the idea was to move most of the functionality from the menubar into more tool options, or into smaller context-specific popup menus. I think if someone was to do a more extensive design review then that would be possible. But it would be a lot of work and changes to put on the users. And likely users would still want the menubar on MacOS where it's required. A good example of a really complex app I've seen that does it right is Blender. Siril is interesting for a GTK app, there is also GNOME Builder.
It's possible to move some of the nested menus into the main window. But as soon as you go anywere near filters, all hell breaks loose :)