HDMI display dims after changing resolution to default

Test a different display with the Mac mini.

Try a different HDMI input port on the Sony TV.

Check the resolution and stand-by settings in the Sony TV's menu.

Replace the cable with shortest possible high speed certified 8K HDMI cable.

for example: https://www.amazon.com/8K-HDMI-Cable-Highwings-Braided/dp/B08M9HND4F/ref=sr_1_1_sspa?

Did the TV come with a remote? If so click on the Settings button on the remote, go to Brightness and increase the brightness there. See if that will do the job.

The 1080p resolution is what you should be using for smaller HDMI Displays and TV's.

What display are you using with the 2014 Mac mini..?

Is the display connected directly to the 2014 Mac Mini or to the EveTV..?