As described, you iPad battery may be reaching the end of its usable life. Whilst connected to the iPad Power Adapter, are you able to use the iPad?
If so, you would be well advised to directly contact Apple Support - and request a remote battery diagnostic. The outcome of this test will inform your next steps. You can contact Apple Support using the Get Support link at top-right of this page.
Alternatively, you might take your iPad along to the Genius Bar at your local Apple Store - where the technicians will be able to test and assess your iPad.
You can also open a support case using the Apple Support App. If not already installed on your iPad, assuming that your iPad is still usable whilst connected to its Power Adapter, the App can be downloaded from the App Store:
https://apps.apple.com/gb/app/apple-support/id1130498044