Wasted quite some hours until I found a working Modeline in this stack exchange post so the ThinkPad works with a HDMI attached Samsung QHD display.

Internal display of the ThinkPad is a FHD display detected as eDP-1, the external one is DP-3 and according to the packaging known by Samsung as S24A600NWU. The auto deteced EDID modes for QHD - 2560x1440 - did not work at all, the display simply stays dark. After a lot of back and forth with the i915 driver vs nouveau vs nvidia/nvidia-drm with and without modesetting, the following Modeline did the magic:

xrandr --newmode 2560x1440_54.97  221.00  2560 2608 2640 2720  1440 1443 1447 1478  +HSync -VSync
xrandr --addmode DP-3 2560x1440_54.97
xrandr --output DP-3 --mode 2560x1440_54.97 --right-of eDP-1 --primary

Modelines for 50Hz and 60Hz generated with cvt 2560 1440 60 did not work, neither did the one extracted with edid-decode -X from the hex blob found in .local/share/xorg/Xorg.0.log.

From the auto-detected Modelines FHD - 1920x1080 - did work. In case someone struggles with a similar setup, that might be a starting point. Fun part, if I attach my several years old Dell E7470 everything is just fine out of the box. But that one just has an Intel GPU and not the unholy combination I've here:

$ lspci|grep -E "VGA|3D"
00:02.0 VGA compatible controller: Intel Corporation CometLake-H GT2 [UHD Graphics] (rev 05)
01:00.0 3D controller: NVIDIA Corporation GP107GLM [Quadro P620] (rev ff)