Well, the system and data partitions are LVM Logical Volumes formatted with EXT4. As EXT4 can be both shrunk and grown, it shoulw be possible to first shrink the data LV filesystem, then shrink it's LV to free up space in the Volume Group (the volume group is full by default). Then just make the system LV bigger and grow it's filesystem.

The only issues should be that while you can grow a mounted EXT4 filesystem, you can only shrink an EXT4 filesystem that is not mounted. So it would have to be done from a recovery shell or something similar.

Another possibility might be to create another Physical Volume on a uSD card partition and add it to the VG, to add more space. Then resize the system LV and then grow it filesystem. But note that by this the uSD card basically becomes part of the - part of the system partition data will be located on it and the device might not boot if it can't find the card at boot time (or if the card dies). Or it might not boot anyway if for example the card shows up much later than the internal storage during boot or if the initrd has some issues finding the PV on the card during boot.

The shrink-data LV, grow-system LV method should be definitely the safer one. And still always backup first before attempting anything like this!