Some apps fail to load
Some apps have started to fail opening
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OK....so this is why you're having issues. The apps should work just fine on your iPad. But you're running out of space on the iPad. And buying more iCloud storage does NOT increase the physical memory on your iPad at all. iCloud is a syncing service, which let's you store more data in the cloud. It has nothing to do with the physical memory on an iPad, which is fixed at the space you purchased.
You'll either need to invest in a newer iPad with more physical memory OR delete things off the iPad you don't need or use, such as photos, music, videos, apps, text messages, email, files, etc.
When you delete photos, they don't immediately delete. They go to a recently deleted folder, where they remain for 30 days to give you the opportunity to restore them if you change you mind. To permanently delete them, you delete them, then go to the recently deleted folder and delete them once and for all.