Screen backlighting issues will not cause appearance of lines on the display or other screen artefacts.
You might be well advised to seek assistance directly from from Apple Support - and request a remote diagnostic test of your iPad. You can contact the Support Team using the Get Support link at top-right of this page. Alternatively, you can initiate and manage your support cases using the excellent Apple Support App. If not already installed on your iPad, the App can be downloaded from the App Store:
https://apps.apple.com/gb/app/apple-support/id1130498044
Given the nature of the problem, the iPad might best benefit from in-person testing by an Apple technicians at the Genius Bar of your local Apple Store or Apple Authorised Service Provider (AASP):
Genius Bar Reservation and Apple Support Options - Apple