@herf This solution worked perfectly! Thank you so very much!

Update: the current dev version of flux (3.11) fixes the flickering issue! Just in time too, since school is just started back up. So now I can use flux on my main monitor! Sadly there still isn't support for USB adapters, but maybe one day... Thank you flux team for fixing this issue. This change will have a huge positive impact on my sleep/health/academic quality.

Forgot to mention that the old laptop was attached to these same two monitors through the Dell docking station, and both monitors dimmed as expected with f.lux. Also, when connected to the dock, the laptop is generally closed, so the laptop monitor is turned off, and the Dell monitor (Monitor #1) is the main screen. Uninstalled and reinstalled f.lux, and expanded the color range again. Still doesn't work.