I want to have a desktop environment / window manager which is powerfull / customizable.
So, kind of, everything at once, huh? Can you define "powerful"? Customizable to what extent? It really doesn't matter whether it's a full-blown desktop environment or just a standalone window manager?
For the full-blown desktop, I personally prefer KDE (Plasma 5). Reason: It's really well-integrated, looks and feels nice, and has a reasonable amount of configurable options (in contrast to e.g. Gnome).
Recently, I decided to take a step back and use FVWM on my desktop, because it's getting old and instead of buying more powerful hardware, let's first see what can be done at the software end of things. FVWM is what I would call powerful and customizable, but here, this means editing (potentially huge) config-files, so you *will* invest some time until you're satisfied. I'm very happy with the speed as well.
Here's the latest result:
I'm using the following additional software with it, so it has enough "desktop functionality" for me:
x11/xscreensaver for screen saving, locking and energy saving
sysutils/gkrellm2 for a few stats like CPU load and email notification
x11-wm/picom as a compositing manager with a nice blurred-translucency feature (unnecessary eye-candy)
x11/stalonetray as a "systray" in my FvwmButtons-based "taskbar", e.g. for kteatime, pidgin, etc
x11-clocks/xclock the "taskbar" also needs a clock in the corner I'm used to looking for it
x11/eterm for Esetroot which sets a background image in a way that works with compositors.
x11/rxvt-unicode as a terminal emulator