Can't change brightness on Kubuntu

After a few days away from Linux, I now realise I'm suddenly unable to adjust the brightness, both via the Fn keys on my laptop as well as by changing brightness in the Energy Savings settings screen.

I've got Kubuntu 20.04 (LTS) with KDE Plasma 5.18.5. I have a Lenovo Legion laptop with NVIDIA GTX 1650.

I've done the usual round of Googling, and tried a few things including this and this, but nothing worked. Running some of the commands in the other answers, I noticed I've got two graphics devices listed:

~$ sudo lshw -C display
  *-display                 
       description: VGA compatible controller
       product: NVIDIA Corporation
       vendor: NVIDIA Corporation
       physical id: 0
       bus info: pci@0000:01:00.0
       version: a1
       width: 64 bits
       clock: 33MHz
       capabilities: pm msi pciexpress vga_controller bus_master cap_list rom
       configuration: driver=nouveau latency=0
       resources: irq:130 memory:c4000000-c4ffffff memory:b0000000-bfffffff memory:c0000000-c1ffffff ioport:4000(size=128) memory:c3000000-c307ffff
  *-display
       description: VGA compatible controller
       product: UHD Graphics
       vendor: Intel Corporation
       physical id: 2
       bus info: pci@0000:00:02.0
       version: 05
       width: 64 bits
       clock: 33MHz
       capabilities: pciexpress msi pm vga_controller bus_master cap_list rom
       configuration: driver=i915 latency=0
       resources: irq:129 memory:c2000000-c2ffffff memory:a0000000-afffffff ioport:5000(size=64) memory:c0000-dffff
~$ lspci | grep -i vga
00:02.0 VGA compatible controller: Intel Corporation UHD Graphics (rev 05)
01:00.0 VGA compatible controller: NVIDIA Corporation Device 1f99 (rev a1)

I'm not sure what else I can try. Could anyone point me in the right direction?