Hi, minhundgarp,
I understand you’re concerned about your mini not having enough storage to have photos stored there. But did you know, where you share photos among devices, the photos can be stored in only once place?
If you want to have them stored on iCloud, where they’re accessible on every device, have iCloud photos turned on. Similarly, if you don’t want them shared, turn this setting off.
Next, learning about the differences between storage on your device and iCloud might be useful: https://support.apple.com/en-us/HT206504
Additionally, here’s how to Manage your Photos and Storage: https://support.apple.com/en-us/HT205703
Lastly, should you decide to keep your photos on iCloud so you can share, here’s How to Manage your iCloud Storage: https://youtu.be/01YAV9_4h4s
Where you store photos can be managed, separately, on each device in Photo’s Settings.
Hope this helps!