Display brightness cannot be changed via keys, Control Center, System Preferences, or Terminal.

This question concerns my mid-2014 Macbook Pro 11,2.

At some point late last year I updated the computer to Big Sur 11.6 from a previous version of Big Sur, and immediately noticed that I could no longer control the screen brightness via any method, and that the brightness was stuck at around 50-75%. The keyboard brightness controls don't have any effect, the brightness slider in the Control Center is grayed out, and there is no brightness control at all in System Preferences. Attempting terminal commands to change the brightness also produced no result.

-I did multiple SMC and PRAM resets.

-Reformatted the SSD and performed a clean install downgrade back to an older version of Big Sur from USB.

-Tried that again, this time also deleting the recovery partition if my memory is correct - this was in December.

-Updated back to Big Sur 11.6.

I'm attaching screenshots of what the Control Center and System Preferences show.

Nothing I've tried has had any effect, and the display brightness still cannot be controlled. The computer has sat for months now because I replaced it with a new M1 Pro Macbook Pro in December, and my old laptop became a low priority. It bothers me though to have an otherwise perfectly functional Macbook Pro that was broken by a software update. Is there anything else I can try as a computer-savvy person that doesn't involve paying for a repair at the Apple Store? (I don't know if the Apple store would even work on the laptop, as it is outdated and has had non-Apple repairs made to it in the past.) My best guess is that fateful Big Sur update last year corrupted something in the firmware, but I don't know how to troubleshoot that.

Thanks.