By default, iCloud Photo Library only downloads thumbnails of your photos onto each individual device. When you view a photo, a device-optimized version is downloaded. Higher-resolution versions may be pulled down as you zoom in. The full-resolution version isn’t downloaded until you try to Edit or Share the photo.
This default setting is best on space-limited devices such as an iPad or iPhone. As I mentioned, only thumbnails are downloaded by default, so you will not fill up your iPad’s storage. As your device runs out of space, iOS/macOS will automatically purge local copies and retain only thumbnails, as needed.
When you enable iCloud Photo Library, make sure
Optimize iPad Storage is checked under Settings → Photos & Camera:
On your Mac Mini, open Photos and make sure
Download Originals to this Mac is selected under Photos → Preferences → iCloud:
The above configuration will cause your Mac Mini to keep a full-resolution copy of all your photos, with only thumbnails downloaded to your iPad by default, and everything backed up in iCloud.
Be aware that these photos will consume your iCloud data allowance. You may need to subscribe to an iCloud storage plan.