I'm having the same issue with a customer's device. Very dim screen, but the brightness keys or the options in Windows are not there. I believe that it has to do with graphics driver, as it's showing with a warning message in device manager. However, in S mode you can't install drivers from the web and the "update driver" option fails every time either from Device Manager, or through Windows update (probably due to the low storage).
Not even trying to reset Windows works.
What a frustrating piece of machine! Any thoughts would be appreciated.
(Btw, I tried reinstalling Windows 32-bit because it should be smaller so it doesn't get the drive too full, but I found out that HP doesn't offer drivers for 32-bit Windows, so the trackpad doesn't work, nor does screen brightness still... 🤦🏻♂️)
I'm trying 64-bit and see how it goes...