Locked into runlevel 3 after installing nvidia-driver

I have a Dell Precision 5820 Tower with an Nvidia NVIDIA GP107GL [Quadro P400] 
card (3 DisplayPorts) and two iiyama monitors in portrait mode. I run kernel 
6.12.90+deb13.1-amd64.

For a long-forgotten reason, this machine boots into run level 3 (multi-user).  
It was suffficient to type telinit 5 to get to the graphical interface.

When I installed Debian 13, the nouveau driver was not able to handle the two 
monitors, so I worked with one only.  Finally I got tired of this and loaded 
nvidia-detect which told me that the meta package nvidia-driver would support my 
card.

So I installed nvidia-driver (66 packages including g++). I then rebooted into 
run level 3 and saw the nvidia driver:

  root@titan ~ inxi -v8  | grep -A2 Graphics:
  Graphics:
    Device-1: NVIDIA GP107GL [Quadro P400] driver: N/A alternate: nvidia
      non-free: 550-570.xx+ status: current (as of 2025-04; EOL~2026-12-xx)

I typed telinit 5 and nothing happened - still in run level 3. I tried systemctl 
isolate graphical.target but nothing happened, I was still in run level 3.  
Command systemctl get-default says "multi-user" so I ran systemctl set-default 
graphical.target and rebooted.

Still in runlevel 3. so I tried to enable graphical.target and got:

root@titan ~ systemctl enable graphical.target
The unit files have no installation config (WantedBy=, RequiredBy=, UpheldBy=,
Also=, or Alias= settings in the [Install] section, and DefaultInstance= for
template units). This means they are not meant to be enabled or disabled using systemctl.

Possible reasons for having these kinds of units are:
• A unit may be statically enabled by being symlinked from another unit's
  .wants/, .requires/, or .upholds/ directory.
• A unit's purpose may be to act as a helper for some other unit which has
  a requirement dependency on it.
• A unit may be started when needed via activation (socket, path, timer,
  D-Bus, udev, scripted systemctl call, ...).
• In case of template units, the unit is meant to be enabled with some
  instance name specified.

Here is /usr/lib/systemd/system/graphical.target :

  [Unit]
  Description=Graphical Interface
  Documentation=man:systemd.special(7)
  Requires=multi-user.target
  Wants=display-manager.service
  Conflicts=rescue.service rescue.target
  After=multi-user.target rescue.service rescue.target display-manager.service
  AllowIsolate=yes

Systemd's graphical.target is loaded and active :

  root@titan ~ systemctl status graphical.target
  ● graphical.target - Graphical Interface
     Loaded: loaded (/usr/lib/systemd/system/graphical.target; indirect; 
             preset: enabled)
     Active: active since Thu 2026-06-18 10:55:44 CEST; 6h ago
     Invocation: 5e9208bc4b264e78b56e3f2489e16903
     Docs: man:systemd.special(7)

This output has colour code green.  Does this mean that the graphical interface 
is running somewhere, or merely available to run ?

Any suggestion or hint to get out of run level 3 would be very welcome, Roger