I just bought 200 gb, but my iPad still says I have only 16 and have used 15.7 of it, which is why I bought another 200.

What you bought was probably 200 GB external storage plan which you pay per month. External storage plan, I mean the storage that sits miles and miles away in a data centre, but not your storage in your IPad mini.

Settings> Ipad Storage > tells you the storage actually inside your IPad. This internal storage is unchangeable. It is fixed, it is always 16GB. The 15.7GB is your current usage of your data, apps and IOS itself. Since you have a 16GB storage/capacity IPad, then have a look how many photos, books, messages, videos that you can delete and/or move to your paid 200GB storage which is called iCloud storage. This 200GB storage is physically in a data centre, so you need to have a Wifi and cellular connection to it.

Sorry, but moving stuff to iCloud isn't going to help. iCloud is a syncing/sharing service, not an archival one. Anything you put on iCloud will also be on your device with few exceptions. So, you don't "move" things to iCloud, you essentially copy them there so that the data in iCloud can by shared by all of your other devices using that same iCloud account.

If you are having an issue with a large amount of your memory being taken up by Other, then the best thing to do at this point would be to erase your device and set it up manually (i.e., not from a backup to start with). If that clears out the Other issue, then you can try to restore it from a backup to see if it remains low. If it goes back up, you will have to erase the device again, and set it up from scratch. You can sign back into iCloud, and all of your iCloud-sync'd data will re-download, and you can download all of your past purchases: Download past purchases.

How to erase your iOS device - Apple Support

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

Best,

GB

If you need to free up free data storage space on an iPad/iDevice, you need to start by backing up your iPad to either Apple's iCloud or by connecting your iDevice to a computer running the latest iTunes or some other offsite, “cloud” data storage service, like Google Drive/Photos, DropBox, Box, etc.

Then delete any installed apps that you no longer use or use infrequently enough that they aren't missed (you can always reinstalled any purchased or free apps at some time later, as you need them).

Thin out/delete any music/videos/movies/TV shows/images/eBooks/eMagazine/iBooks/audio books/Documents/PDFs, etc.

If you use a POP email account on your iPad, delete/thin out any old saved emails that maybe saved to your iPad.

To permanently delete photos/images off of your iDevice go into the Photos app and find the Recently Deleted folder/album and permanently delete any photos/images stored in that folder/album.

Also, you can check to see what stuff is eating up all your valuable data storage space by going into the Settings App,,General Settings Panel, in the right hand column, tap Storage & iCloud usage panel ( in iOS 11, it’s iPad Storage ), in the top Storage Panel, tap the Manage Storage panel, wait for a list of apps to generate that will tell you how much space each app is taking up on your iDevice.

Then, do a hard reset of your iPad by holding down both the Home and sleep/wake buttons simultaneously until your iPad goes to black and restarts with Apple logo, then release the buttons.

iDevices need to maintain, at the very least, a minimum of between 2-3 GBs of free data storage space, at all times, to insure normal/nominal functionalities/operation and general overall performance of any iDevice.

Good Luck to You!