No HDMI signal on TTY7 when running Buster + XFCE + NVIDIA GT 640 + nouveau + 4K UHD

Symptom
Upgrading from a WQHD ViewSonic display to a giant 4K UHD ViewSonic display yields a No Signal error (via HDMI) after my workstation successfully makes it through GRUB and Plymouth screens. The Debian-branded Plymouth splash screen seems to display at 4K, but then it never displays the LightDM greeter; the new monitor loses the signal at that point.

I can switch over to TTY1 where that TTY seems to be running at 4K, based on tiny font size and its sharpness. Furthermore a forest-style ps reveals that the greeter is hanging off a TTY7 X process:

Code:root 810 1 0 59886 9164 2 16:09 ? 00:00:00 /usr/sbin/lightdm
root 817 810 0 138738 82472 2 16:09 tty7 00:00:00 \_ /usr/lib/xorg/Xorg :0 -seat seat0 -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch
root 843 810 0 41710 7412 2 16:09 ? 00:00:00 \_ lightdm --session-child 16 19
lightdm 858 843 0 196049 133484 2 16:09 ? 00:00:01 | \_ /usr/sbin/lightdm-gtk-greeter
root 897 810 0 4687 5112 0 16:09 ? 00:00:00 \_ lightdm --session-child 12 19Configuration
The sole GPU:

Code:01:00.0 VGA compatible controller: NVIDIA Corporation GK107 [GeForce GT 640] (rev a1) (prog-if 00 [VGA controller])
Subsystem: eVga.com. Corp. GK107 [GeForce GT 640]
Flags: bus master, fast devsel, latency 0, IRQ 27
Memory at f6000000 (32-bit, non-prefetchable) [size=16M]
Memory at e0000000 (64-bit, prefetchable) [size=256M]
Memory at f0000000 (64-bit, prefetchable) [size=32M]
I/O ports at e000 [size=128]
Expansion ROM at 000c0000 [disabled] [size=128K]
Capabilities: [60] Power Management version 3
Capabilities: [68] MSI: Enable+ Count=1/1 Maskable- 64bit+
Capabilities: [78] Express Endpoint, MSI 00
Capabilities: [b4] Vendor Specific Information: Len=14 <?>
Capabilities: [100] Virtual Channel
Capabilities: [128] Power Budgeting <?>
Capabilities: [600] Vendor Specific Information: ID=0001 Rev=1 Len=024 <?>
Capabilities: [900] #19
Kernel driver in use: nouveau
Kernel modules: nouveauAnd

• Debian 10
• Kernel Debian 4.19.132-1 (2020-07-24) x86_64 GNU/Linux
• lightdm 1.26.0-4
• xfce4 4.12.5
• xserver-xorg 1:7.7+19

Troubleshooting
Verified that the new monitor can be driven via HDMI at 4K by connecting it to a notebook running Pop!_OS + Gnome.

Back on my main workstation I collected various LightDM, X11, and XFCE config and log files after booting separately with the WQHD and 4K monitors, looking for a smoking gun.

X11 seems to try to drive the monitor at 4K,

Code::~/troubleshooting/xfce_at_4k_uhd$ egrep -C2 "3840.+2160" *
Xorg.0.log-[ 26.169] (II) modeset(0): 0000000000000000000000000000001a
Xorg.0.log-[ 26.169] (--) modeset(0): HDMI max TMDS frequency 600000KHz
Xorg.0.log:[ 26.169] (II) modeset(0): Not using default mode "3840x2160" (bad mode clock/interlace/doublescan)
Xorg.0.log-[ 26.169] (II) modeset(0): Not using default mode "2560x1440" (bad mode clock/interlace/doublescan)
Xorg.0.log:[ 26.169] (II) modeset(0): Not using default mode "3840x2160" (bad mode clock/interlace/doublescan)
Xorg.0.log:[ 26.169] (II) modeset(0): Not using default mode "3840x2160" (bad mode clock/interlace/doublescan)
Xorg.0.log-[ 26.169] (II) modeset(0): Printing probed modes for output HDMI-1
Xorg.0.log:[ 26.169] (II) modeset(0): Modeline "3840x2160"x60.0 533.00 3840 3888 3920 4000 2160 2163 2168 2222 +hsync -vsync (133.2 kHz d)
Xorg.0.log:[ 26.169] (II) modeset(0): Modeline "3840x2160"x24.0 297.00 3840 5116 5204 5500 2160 2168 2178 2250 +hsync +vsync (54.0 kHz e)
Xorg.0.log:[ 26.169] (II) modeset(0): Modeline "3840x2160"x24.0 296.70 3840 5116 5204 5500 2160 2168 2178 2250 +hsync +vsync (53.9 kHz e)
Xorg.0.log:[ 26.169] (II) modeset(0): Modeline "3840x2160"x30.0 262.75 3840 3888 3920 4000 2160 2163 2168 2191 +hsync -vsync (65.7 kHz e)
Xorg.0.log-[ 26.169] (II) modeset(0): Modeline "3200x1800"x60.0 492.00 3200 3456 3800 4400 1800 1803 1808 1865 -hsync +vsync (111.8 kHz d)
Xorg.0.log-[ 26.169] (II) modeset(0): Modeline "3200x1800"x59.9 373.00 3200 3248 3280 3360 1800 1803 1808 1852 +hsync -vsync (111.0 kHz d)
--
Xorg.0.log-[ 26.170] (II) modeset(0): Output HDMI-1 connected
Xorg.0.log-[ 26.170] (II) modeset(0): Using exact sizes for initial modes
Xorg.0.log:[ 26.170] (II) modeset(0): Output HDMI-1 using initial mode 3840x2160 +0+0
Xorg.0.log-[ 26.170] (==) modeset(0): Using gamma correction (1.0, 1.0, 1.0)
Xorg.0.log-[ 26.170] (==) modeset(0): DPI set to (96, 96)But it's unclear to me where things go wrong. Can anybody suggest what to look for, or what else to post here as clues?

Thanks