Pingoshea basically you need to free up space on your ipad, get rid of all the stuff you dont need, then remove all your backups then hopefully you will have enough space to do a full backup.
I have a 256gb ipad and i am struggling for space , but sometimes you just have to declutter the device , i know its hard as you become an app haurder but you just have to be brutal.
I have an ipad dedicated to ST3, anything else is secondary in the order do i need it to support my ST3 software , if not its gone, so even a 128 gb device should be fine.
At the moment i have a 30gb full backup in theory i could even use a 64 gb device but in reality i definitely need a 256 gb device as there are so many other apps that compliment the ST3 its hard to get rid of them.
So either clean up or upgrage to bigger hard drive device.
Good luck with the clean up.
Cheers Damir