Somewhat agree, but don’t get me started on a Gimp. To think that gimp was build to be a tool analogous to Photoshop (PS) is naive. It was born to demonstrate GTK GUI widgets and to check boxes on feature list (of supposedly paint program analogous to PS) from programmers perspective at most. Ok, they did the thing, checked the boxes, used all widgets, demonstrated that it works and from that day on it had and still has totaly inneficient workflow compared to PS and nobody cares about that. Answer to sugestions is almost always half assed, apple soused - you are holding it wrong, we are not PS. :)
My 2 cents, you can learn Gimp, you can adjust yourself to it, but if you have ever worked on PS and were good at it (with all its workflow, shortcuts, up to the level where you work one hand on keyboard, having most toolboxes hiden out of your view, etc…) you’ll still feel gimpy. It’s like comparing of giving commands to the gnome with an axe versus to an elf with a whole bunch of efficient specialised tools, spells and workflows – both trying to create art. I don’t use PS daily for how much, maybe >8 years and use Gimp weekly for about 12years – I say, it is still gimpy as f… And I’m programmer not a designer, designers usualy just hate it. I on another hand understant it (and it’s history) and take it as it is, as an inferior gimpy cousin of PS :)
Easyest way, to begin with, is to use a browser addon and userscripts. Addons to look for are Greasemonkey, Tampermonkey, or Violentmonkey and alike alternatives. Keyword to google “userscripts”. P.S. As I see it, from your description, you just want some mods (changes) to existing UI, no need to rebuild one from a scratch in other more complicated ways.