Have you looked at your battery drainage stats in Settings yet? Take a look there and see if anything unusual is draining the battery an excessive amount in the background. Can you also list what version of iPadOS you're using currently? I believe there's been an update or two since you got your iPad.
Unfortunately though, I have had your exact same problem on three different iPads in the past. It always seemed to come and go with dot releases of iPadOS. For example 13.3 would make my iPad Pro drain 25% overnight in standby if I didn't turn its wifi off before I went to bed, but then maybe 13.3.1 would fix it, then maybe 13.3.4 would reintroduce it, etc. There were even a couple of months when the Pencil 2 hadn't been out very long where the Pencil itself was draining in standby, even while connected to the side to charge.
I've had this issue on the 10.5" iPad Pro, the 2018 11" iPad Pro, and the 2020 12.9" iPad Pro. Never for long, it was usually resolved by the next dot release, but I also never figured out how to fix it other than turning off the iPad's wifi radio overnight so it couldn't chug away on whatever it was doing while I was sleeping. I tried everything from a simple force quit all apps and hard reboot to a full blown DFU restore while connected to a Mac and then not installing any apps at all afterwards. Not even that ever fixed it on any of the models. 2 or 3 days later, even just letting it sit with stock setup out of the box on a freshly downloaded and installed OS, the problem would return.
My current iPad, the mini 6, does not have this issue on the latest public release of iPadOS, which is why I'm so curious about what version your device is on.