Inkscape's interface isn't too bad, not good, but I rarely have significant issues with it. GIMP is an absolute nightmare of UI/UX design, I can't understand how someone hasn't fixed that by now.
The issue, as you point out, is lack of designer. Both Qt and GTK can be extensively customised, but you need to know what you're doing (in an artistic sense).
Of course you can understand that. Try writing actual code to fix any pet peeve with GIMP's UI/UX (or any app's UI/UX). Then we'll sit down and talk again about the role of volunteers in a free software project :)