Your low storage, 16 GBs iPad is FULL!
A 16 GBs storage iPad starts out, after initial drive formatting, at approx. 14.5 GBs of free, internal storage.
After the installation of iPadOS, at approx. 6 GBs, leaves 8.5 GBs of of free, internal data storage space.
iOS/iPadOS needs a minimum of 2-4 GBs of constant remaining storage, that MUST BE MAINTAINED AT ALL TIMES, to insure/ensure nominal, acceptable operation of all iOS/iPadOS functionalities.
This leaves a 16 GBs iPad with ONLY a practical, usable storage space of between 5 and 7 GBs AND THAT IS IT!
This is virtually NO internal data storage to do much of anything with that iPad!
This is why Apple quietly, silently did away with all 16 GBs iOS device back sometime in 2016.
There is NOT really anything that can be done to better your storage issues on that small storage iPad model.
Compare this to a 32 GBs iPad model ( which, in 2021, is now quickly becoming the “new”, low data storage 16 GBs iOS/iPadOS device. )
A 32 GBs storage iPad starts out with approx 30 GBs.,after initial internal flash drive formatting.
Minus approx. 6 GBs for iOS/iPadOS leaves 24 GBs.
Minus 2-4 GBs, for iOS/iPadOS system operational reserve, that MUST BE MAINTAINED AT ALL TIMES, to ensure/insure normal, expected operation of iOS/iPad OS, leaves approx. between 20-22 GBs of actual, usable data storage space.
This is 3-4 times the free, internal data storage space of a 16 GBs model iPad.
It's 2021, now.
You should really consider a purchase of a new iPad model OR a much, MUCH “newer” and less expensive, refurbished or used iPad models, from better known, trusted sources, like other Apple product/device retailers/resellers, locally or online, on the Web/Internet, capable of running the latest iPadOS versions, that offer much larger internal data storage space, now.
There are PLENTY of newer, older iPad models that can run the latest, current versions of iPadOS and have either 32
Sorry & Best of Luck to You!