The overall setup is X11 with a Sawfish window manager. X11 provides a lot of cool features like network transparency and automation and generally being wildly flexible. Sawfish is like the Emacs of window managers, written in a flavor of Lisp, and you can literally rewrite its code while it's running. So whenever I want something to work differently... I just find that part of the code, change it, and activate it.
If I manage to crash it, that's fine... Sawfish isn't the outer process for my session. For that, I have my .Xsession just do a simple loop. It keeps running until I touch a file called ~/.xlogout, and can restart the window manager or other tools if necessary.
I pretty much never log out or reboot unless it's time to do a dist-upgrade... which means once every couple years on average. Everything just stays running.
The main limit is that Xorg is usually compiled to use an 8-bit ID for each client, so it can only have 255 clients connected at once, which usually means a limit of 255 windows open at a time. But you can push this a bit using programs which do multiple windows in a single client connection, like "urxvtcd" in rxvt-unicode can have hundreds of terminals sharing just one client slot. I've occasionally had 500+ windows open on a single computer (though this is kind of a bad habit, and I try to keep it under 200).
I configured Sawfish to do soft tiling, and also to do tabbed windowing, and made all of it work via keyboard, including things like moving and resizing windows. Sawfish is also where I implemented the "every flavor of F1-F12" hotkeys thing, and implemented mouse warp, and it already had a lot of things built in like sloppy focus and the ability to add and remove desktops with a keypress.
I added another thing to it to save the contents of every desktop, every window, size and position and title and tab grouping, etc... every 15 seconds or so if it has changed since last time. This gets archived daily. So if the whole system crashes, or if I need to actually log out, or if something bugs out, I can restore things mostly how they were.
A lot of the stuff I don't have, like a task bar, or a desktop notifications system, is simple enough to do by just not installing one or not running any of the relevant programs or services.
As for traditional-ish desktop stuff, there's not much. I set the wallpaper using a small program I made decades ago, which basically just picks a file at random from a big nested directory structure of images I've accumulated over the years. I use a 2D grid style desktop pager in the corner of the screen to see which desktop I'm on, with Hyper+Arrows keys to navigate the grid, but I barely ever look at the widget and it sticks to the bottom stacking layer so other programs are free to draw over it. Similar story for a Conky instance configured in a thin vertical strip with all sorts of realtime system stats and any other info I care to put there. It's a quick, easy way to see what's going on, but I often cover it up and use that space for other things. It's basically just system stats rendered onto the wallpaper.
I mostly use keyboards with open-source firmware like QMK, with a pretty extensive personal keymap tailored to my needs and tastes. I also added some stuff to QMK, like a better MouseKeys motion mode called "inertia" mode, to make mouseless use easier. But on notebooks and legacy keyboards, I can at least use Kanata to get most of the same features.
Macros recording/playback is built into the keyboard, or X11 in general is pretty easy to automate with small programs or shell scripts or even one-liners. Like, with xdotool.
For remote controlling other systems, x2x is great. Ancient program which "just works". Or running remote programs on the local screen, ssh + normal X11 stuff. Or for a full remote desktop, there's VNC (there's a tigervnc xorg extension, and the tigervnc viewer is good too... I patched it to make it handle larger desktops better, and the patches are upstreamed).
(continued in next comment, my post was too long to fit in one comment)