It really depends on what you want to use your device for.
9-10GB of space is barely enough to install new versions of iOS/iPadOS, or particularly heavy applications, without considering that each download/installation/use requires taking up additional space to index data correctly.
Without wanting to go into details, I believe that the free space for good efficiency of the device must always be greater than at least 25%-30% of the overall capacity.
You will undoubtedly have noticed that almost all devices today are sold with a capacity of at least 128GB (those with 16, 32, 64GB are no longer marketed), and this says a lot about the importance of the space occupation of new operating systems , apps, data, and media that increasingly require space if you want to obtain quality results (think of the high resolutions of photos and videos that can be had today)
It is therefore difficult (and very subjective) to recommend an optimal overall capacity.
I hope that this answer (certainly superficial) can still make you understand the importance of always having as much free space as possible available.
Regards
Giulio
P.S. I really appreciate that you also understood the need to create backups (or partial saves of some media) on a regular basis, to make room on your iPad. Mobile devices are now real computers and, as such, contain too much information which, for the user, is of vital importance.