That command will bypass your ~/fluxbox/startup file (has always done that for me, but VSIDO is set up a bit different)
is the usual.
Or, a $HOME/.xinitrc:
# optional startup stuff, edit to suit your needs
xrdb -l $HOME/.Xresources
fbsetbg -c -r $HOME/wallpaper & <- sets a random wallpaper from given directory
and then startx from the console. No login manager, but you'll need to add xinit package to iso. Change the "exec flubox" line to whatever is needed in Debian for automounting if you want/need that.
I like lxdm -
1. more resource friendly than lightdm
2. still get the automounting (because I'm lazy)
3. select from multiple window managers
4. /etc/lxdm.conf is easy to set up for aesthetics
If you want a login mnager to get to X, you can use one of the old faithfuls like xdm and custom VSIDO .xinitrc. That should be an easy setup. Xdm doesn't read the /usr/share/xsessions/*.desktop files, but you only ship one window manager so no big deal. Only issue is whether or not it works with systemd.
If users want multiple window managers, lxdm or lightdm etc ... are just an apt command or two away. Any one looking to try Sid is most likely aware of that.
I actually switched to lxdm on VSIDO installs a while back becuase lightdm was always getting borked by systemd upgrades. Was happy with it from the start. Just have toi set up apt to not install all the recommended stuff from lxde.