Data partition with 23GB only shows 8GB when mounted
After "playing" a bit with the device, I managed to trace the problem to the ext4 partition data. Booted into TWRP, terminal, backed up the files in the partition, unmounted, then:

mke2fs -t ext4 /dev/block/bootdevice/by-name/userdata


And voila! 23GB of data storage available. Restored the backed up files to the new ext4 filesystem and everything is working now, although I feel it a little slower. Nevertheless, it's solved.