iMac display brightness keeps increasing -- Already reset SMC a few times

The display brightness on my iMac 27" 5K keeps increasing incrementally each time I shutdown and turn it back on. I've seen this issue posted several times, but the threads always seem to end with resetting the SMC, which I've tried a few times already. I've also reinstalled Catalina. The problem always returns. Other settings: The automatic brightness option is unchecked; I have the display set at a scaled resolution for better readability; Appearance is set to Dark Mode. I've restarted in Safe Boot mode, but it's hard to troubleshoot the brightness because the "Automatically adjust brightness" bar completely disappears and the scaling options do not work. Any suggestions? Thanks in advance.