Cannot adjust display brightness by any method after update to 11.6

This question concerns my mid-2014 2.2GHz Macbook Pro 11,2

At some point in Novemer 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 - keyboard, Control Center, System Preferences, or Terminal - 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. The display brightness isn't adjustable in any alternate boot mode either - safe mode, recovery mode, or even when booted into an installer from a USB drive.

Things I've tried:

• 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 up to Big Sur 11.6.
• Disconnected the battery.
• Unplugged and reseated the display cables to the logic board.
• I even decided to try out an OpenCore Legacy Patcher Monterey installer I had sitting around to see if forcing a newer os install might refresh something, but the same problem persists.

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 the 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? (I don't know if the Apple store would even work on this laptop, as it is "vintage" and has had non-Apple repairs made to it in the past.)

As an aside, the computer has an aftermarket SSD, and a replacement display installed by me. The replacement display worked flawlessly for about three years before this issue happened, so I don't think it's the fault of the display.

Thanks.