How to configure VNCServer using exactly one installed desktop env (KDE or Cinnamon) on Debian 12 (bookworm)

I’ve been trying to set up a machine with the following requirement:

  1. accessible desktop environment both remote and local
  2. the same desktop environment in both
  3. both can run be in use simultaneously
  4. the local one and the remote one do not have to share the same content (not screen sharing).

The best I get is that the console DE works while the vncserver remote either goes to a grey/black screen when the console is logged in or the console goes back to the login screen w/o getting into the desktop when the remote vncserver is logged in.

Is there some magic I have to put into the xstartup file to make it work?

Right now the machine has both KDE-Plasma and Cinnamon on it. I installed tigervnc per most of the vanilla instructions I found in many various places using ssh to get to the machine:

<console desktop environment is currently using KDE-Plasma (X11)>
<vncserver remote desktop environment is using Cinnamon>
<no xstartup script at all>

dan@gozer-pihole:~$ sudo apt install tigervnc-standalone-server tigervnc-common

dan@gozer-pihole:~$ vncserver
dan@gozer-pihole:~$ vncserver -kill :1

dan@gozer-pihole:~$ vim .vnc/config
dan@gozer-pihole:~$ cat .vnc/config
session=cinnamon
geometry=1920x1080
localhost=no
alwaysshared

dan@gozer-pihole:~$ sudo vim /etc/tigervnc/vncserver.users
dan@gozer-pihole:~$ cat /etc/tigervnc/vncserver.users 
# TigerVNC User assignment
#
# This file assigns users to specific VNC display numbers.
# The syntax is <display>=<username>. E.g.:
#
# :2=andrew
# :3=lisa
:1=dan

dan@gozer-pihole:~$ sudo systemctl start  tigervncserver@:1
dan@gozer-pihole:~$ sudo systemctl enable  tigervncserver@:1

dan@gozer-pihole:~$ sudo systemctl status tigervncserver@:1
? tigervncserver@:1.service - Remote desktop service (VNC)
     Loaded: loaded (/lib/systemd/system/tigervncserver@.service; enabled; preset: enabled)
     Active: active (running) since Tue 2025-01-21 21:25:49 PST; 14h ago
   Main PID: 4894 (tigervncsession)
      Tasks: 0 (limit: 9305)
     Memory: 436.0K
        CPU: 32ms
     CGroup: /system.slice/system-tigervncserver.slice/tigervncserver@:1.service
             ? 4894 /usr/sbin/tigervncsession dan :1

Jan 21 21:25:49 gozer-pihole systemd[1]: Starting tigervncserver@:1.service - Remote desktop service (VNC)...
Jan 21 21:25:49 gozer-pihole tigervncsession[4894]: pam_unix(tigervnc:session): session opened for user dan(uid=1000) by (uid=0)
Jan 21 21:25:49 gozer-pihole systemd[1]: Started tigervncserver@:1.service - Remote desktop service (VNC).

And this all works fine, but I really want the two to be using the same desktop environment, be it KDE or Cinnamon.