When you install an update on an iPad, you need to understand that all the new, update data have to be available to the ipad memory/storage alongside the existing Operating System data. You will need to factor in the size of the update and add that to the existing space available on the iPad in order to make room.
It is possible to download the update to another Apple or a PC - add the free Apple Devices app to a PC. The initial download, unpacking and verification is performed on the connected device.
I found the most helpful instructions in the second half of this link. (You may have to copy and paste the link minus the square brackets).
[https://support.apple.com/en-gb/108327]
I think you have probably realised by now that your iPad will struggle from now on. If you buy a new iPad, the price, to some extent, is dependent on available storage. The cheaper the iPad, the less storage is available. This is actually an imprtant factor in buying any IT device.