Hey Silverisgrey,
We’re going to ask that you take a look at the following article to help check the space in iCloud - Check your iCloud storage on any device As you mentioned you have an iPad, here is a snippet from the article on where to do that -
Check your iCloud storage on your iPhone, iPad, or iPod touch
1. On your iPhone, iPad, or iPod touch, go to Settings > [your name] > iCloud.
The bar graph shows your overall storage usage.
2. Tap Manage Account Storage or Manage Storage for more details.
Below the graph, you see a list of apps and features and how much iCloud storage they use. The apps and features that use the most storage are at the top of the list. You can tap an app or feature for more information.
Once you’re able to identify what is taking the most space, we may be able to determine where things can be freed up to create more.
Also, the prompt may be coming because your device is attempting to make a backup to iCloud and that can be disabled in Settings > your name > iCloud > iCloud Backup.
Thanks.