iPad 10.5in Pro screen constantly dim then bright.

Pending your return, these support pages should help with troubleshooting charging issues:

Charge and monitor the iPad battery - Apple Support

If your iPad won't charge – Apple Support

Troubleshooting generally requires substitution of the Power Adapter and cable for another known-good item.

If you continue to experience difficulties, you might be best advised to seek assistance directly from from Apple Support - and request a remote diagnostic test of your iPad. Results of test may give insight into the overall health of your iPad battery - and may inform you next steps. Detection of display hardware issues may be more challenging.

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

As described, given your reported issues with both your battery performance and display issues, your iPad may have one or more hardware faults. You might be well advised to visit the Genius Bar at your local Apple Store - where the technicians will be able to test and assess your iPad:

Genius Bar Reservation and Apple Support Options - Apple