iPadOS takes up 2/3 of storage space on iPad 128 Gig - 8th Gen - 15.7

Not sure.

After ANY iOS/iPadOS upgrade/update, you should ALWAYS perform a simple hard reset of your Apple mobile device.

For iPad models with a Home button, do a simple hard reset of your iPad by holding down both the Home and sleep/wake/power button simultaneously until your iPad goes to black and restarts with Apple logo, then release the buttons.

Good Luck!

You may need to backup the data on that iPad to Apple iCloud or to a Mac or Windows PC and then completely erase all contents and settings in that iPad, in the Settings app, under the General panel, Rest panel, then setup that iPad as a new device and thrn do a data restore from backup back to that iPad.

Good Luck!