OP iTunes (certainly Mac version) has the best graphic of storage usage. As abover “other” can be huge, often email attachments for me.
I have never had the phantom memory usage but perhaps its because I backup to computer at least once a month
Also just checked as iOS takes about 3GB so if phone is 8 5 left is corrects. Also I posted earlier the wife’s phone was 8 but its 16
Do you have a downloaded but not installed iOS update ?
Anyway iTunes and a sync thrn restore from backup seems like a plan
Finally 😉 … “just add an sd card to android” isn’t true as it depends on phone amd andrpid version. FIL new phone 5.1 ‘driod SD card can’t be used for apps just photo/video