This isn't a brightness autoadjust thing. Looks like your backlight is out.
An LCD screen that is actively displaying an image but with no backlight will still show a faint image from reflected light, so you can see an image in a lit room but not in a dark room.
The only problem with the AMD one is that I'm still waiting for mine.
I've got an older Asus AMD laptop (it's falling apart physically), and it is crazy fast. Zen 2, Zen 3, and Zen 4 have all been fantastic.
Linux runs fine on Arm, doesn't it?
Are you using the AMD accelerated encoder or software encoding?
Ryzen CPU's can absolutely sustain high CPU load for a long time without overheating. I do it all the time.
Zen 4 is designed to be able to sustain very high clockspeeds, limited only by thermals (i.e. it will clock itself as high as it can without going over 95C). But it doesn't have to do this. It turns out that Zen 4 can sustain high performance at a fraction of this power budget -- you'll get something like 80% of full performance at 40% of the power, for instance.
If it turns out that 45W runs too hot, you can limit the CPU to less power and still get excellent performance.
I imagine KDE will work fine, too, and what I intend to run (I'm batch 9...)