I ran into several problems while installing it, and I just want to share the solutions I found for them.

¿RaspberryOS light or full?
Well, the easiest way is to opt for a full flegded OS, but since the Zero W has so limited capability, I opted for a Light install. So I had to adapt my installation mixing several ( outdated ) installation tutorials I found. In the end, I installed Xorg and openbox manually and start them from my .profile with autologin. After that, I start the browser from whithin my /etc/xdg/openbox/autostart

Which lightweight browser can I use on the Zero W?
The quick and easy answer is none. Chromium is not supported anymore on the Zero W ( legacy build is an option though, but segfaults frequently due to the limited amount of ram ); Midory works ok, but I was not able to remove the direction bar from the top, so that did not work either. So in the end, I opted to write a simple “browser” myself with Python Webkit2 bindings. I had to install de following software:
sudo apt-get install gir1.2-webkit2-4.0 libgtk-3-dev libwebkit2gtk-4.0-dev python3-gi

After that, the following file did the trick:

import gi
gi.require_version("Gtk", "3.0")
gi.require_version("WebKit2", "4.0")
from gi.repository import Gtk, WebKit2

class Minibrowser(Gtk.Window):
    def __init__(self):
        Gtk.Window.__init__(self, title="MagicMirror²")
       # The size doesn't actually matter since on the bottom we are going to maximize the window
        self.set_default_size(800, 600)

        web_view = WebKit2.WebView()
        web_view.load_uri("http://localhost:8080")

        scrolled_window = Gtk.ScrolledWindow()
        scrolled_window.add(web_view)
        self.add(scrolled_window)
        self.fullscreen()

if __name__ == "__main__":
    win = Minibrowser()
    win.connect("destroy", Gtk.main_quit)
    win.show_all()
    Gtk.main()

¡The boot process is ugly!
Yes it is and I installed plymouth to make it more attractive. In the end it doen’t actually matter and it even delays de boot for a few seconds, but I like it!
I installed the plymouth spinfinity theme and made it all black. After that I added splash quiet to the /boot/cmdline.txt file and that was it.

I hope that this helps a few fellow tinkerers who wish to repurpose their old Zeros.

Best regards
Merlin