All rechargeable batteries have a limited life - are consumable items - and must eventually be replaced. Given your description and the age of your iPad, it is almost certain that the battery has reached the end of its usable lifespan.
For practical purposes, for battery service of qualifying devices, Apple usually exchange the iPad for a renewed/refurbished iPad of the same model and specification. The cost of the exchange is the same as would otherwise be charged for battery replacement. When exchanged, the replacement iPad is yours to keep.
You can verify both eligibility and cost for your country/region here:
iPad Repair & Service - Apple Support
Exchanging the iPad is a quick and easy alternative - and if exchanged in store, is usually same day. Unlike iPhone, for which an in-store battery replacement might be possible, iPad’s must be sent to Apple’s service partner - this process taking potentially many weeks.