iPad screen dims every 30 seconds

Presumably you have an iPad Pro with FaceID. To alter the observed behaviour, try enabling (or disabling) the Attention Aware feature...

Settings > FaceID & Passcode > Attention-Aware Features - set to OFF

When set to ON - the iPad will automatically dim the display when you are not actively looking directly at the screen - the timeout period being determined by the Auto-Lock setting (Settings > Display & Brightness > Auto Lock).

Are you seeing this issue continuously, or after a period of use? Do you see the problem occurring with a specific App, or all Apps?

Other causes of screen dimming can often be attributed to high screen brightness (in particular when using the Reference Mode) and/or CPU/GPU intensive Apps. These conditions can cause considerable internal heating - causing the iPad's thermal management to protect the iPad's electronics and battery from potential damage.

Thermal management may temporarily reduce screen brightness and/or throttle the CPU until such time as the internal temperature is reduced.