system storage full after migrating to new iPhone, old phone has plenty of free storage
You most likely have selected keep originals in the photos settings.
Open the settings app and scroll down or search for photos. Then under the iCloud section there should be listed the following options:

Optimise iPhone storage
Download and keep originals

Select the first option and your iPhone will offload older photos and only store them in iCloud. Only new photos and often viewed ones will be stored locally, so you can save a lot of space on your phone. Alternatively you can select this option if you go into [your name] -&gt; iCloud -&gt; Photos as described on AppleSupport.