Should I buy a new iPad 8 or replace the battery?

The performance of your existing iPad 8 should not be assumed to reflect that of other models - or even the same model if your current iPad has a fault. Whether or not you choose to replace your iPad with a more recent model, only you can make this decision.

As for your iPad8, you might be well advised to visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to test and assess your iPad and its battery:

Genius Bar Reservation and Apple Support Options - Apple

With iPads, they have no battery health so your having to guess and look for things like: battery draining really fast, battery suddenly dying, charge meter doing weird things (my iPad 7 did that and I think it needs its battery replaced), ( my iPhone 5 was at 80% but suddenly it died this was about 4 years ago)