How do I get more memory for my iPad

You cannot get more memory on an iOS device. What you get when you make the purchase is all you get. If you have filled, or at least almost filled the storage on the iPad, you are going to have to delete some content before you are able to update the device, or try using a computer and do the update. See this, If you need more space for an update on your iPhone, iPad, or iPod touch - Apple Support

You may interest in subscribing iCloud service to store your photos and other files on your Mini. It can help you get your space back from them. Check this out https://support.apple.com/en-us/HT201238

According tho this Apple Support document, your iPad cannot update any further: iPad models compatible with iPadOS 15.1 – Apple Support