I fully expect my mini 6 is reporting the correct used storage overall. It's just the categorization that's off.
Having used iPads (3) and Ipad Minis (3) for ten years (plus various iPhones), I believe this is correct, and agree with you. I have never been able to make sense of the categorization of the storage on any of my iPads, and often it simply does not add up or make sense. In some cases it is reporting near-zero storage for photos, but I can still pull up full resolution photos without any wifi, bluetooth or cellular connections, so it must be storing the photos somewhere on the device.
I think, however, the total storage usage is roughly correct - but this also varies by GBs, both up and down, day to day with no obvious reason.
I conclude the total storage usage is just a rough indicator, and the categorization is only useful to identify apps that are hogging a lot a data storage so they can be purged. Looking at the storage categorization just creates worry and frustration, a bit like obsessing over the memory usage in Activity Monitor on a Mac.
My experience is that 64GB is likely plenty for most people with typical uses for an iPad, provided you use iCloud, optimize photos, and offload unused apps (e.g., get rid of GarageBand). For many years I used an iPad with only 32GB with no issues.
If you don't use iCloud or want to keep all your photos on the device, or have some other special need for data on the iPad, then 64GB is most likely a bad choice.