This is a nice write up.
The dark side is that they've put years into developing the core infrastructure to empower their Lisp game journey. Ideally that's all usable by others to "stand on the shoulders", so to speak, but that's always a challenge.
I honestly have never had to work on a "running image" of any consequence. I've never worked on anything more than a few thousand lines of code. I've never needed any kind of source level "step" debugging. The ability to redefine a function quickly, I just throw in some prints to figure something out.
Reloading the entire source file was always fast enough to now warrant hunting down any of the more interactive mechanisms available. This is all across using simple vi, or emacs, or even LispWorks.
(defun l () (load "src.lisp"))
All that said, my current low level fantasy is something akin to Electron, only with Common Lisp. First class DOM, first class event hooks, into a CL runtime that you can deploy cross platform. Dunno if that exists or not.
I'm sure at some point my codebase will grow to the point where I would need to "level up" and become more intimate with SLIME, packages, system definitions, etc.
I've just not crossed that threshold yet.
Have you checked out https://github.com/rabbibotton/clog ? I have only played with a couple of the tutorials, but it seems to be targetting something like that, albeit without an included browser.