Adjusting brightness on 27" iMac used as external DisplayPort monitor for PC
I just had an issue with not being able to change the brightness on my 27inch iMac in Target Display Mode. It was at full brightness. And the brightness keys on the keyboard hooked up to the TDM Mac would not adjust it. But I found if I pressed and held Commond-F1 on that keyboard it slowly lowered the brightness. This probably won't help much if you have an issue with it defaulting to a low brightness though.

iMac 2009 High Sierra and iMac Pro Mojave.