Available free space on iPhone slowly creeps away

Hi, I have been fighting this problem for a couple of years with my iPhone 5s. It is multi faceted.

1. iTunes will tell me the iPhone has 5 or 6 GB of free space, but the phone really only has 20 MB or so of free space (per setting>storage space, and per the messages that pop up and tell me I am almost out of space.)

2. I've tried literally everything, numerous times, over the years. What works best? Choose erase all "settings and content" and then restore from the latest backup - after I do this the phone will tell me I have around 7 or 8 GB of free space.

3. If I add no apps, and change no setting, over the course of the next month my free space will slowly fade away until once again I have 20 MB or so of free space. Again, iTunes will lie to me and tell me I have 6 GB of free space. This cycle happens over and over.

WHAT I'VE DONE (but does not help):

* Turned off all Photo synching (only the standard photo stream is "on" - and I do not take many pictures)

* Messages only saves 30 days worth - and I delete all pictures in my messages.

* Keep the amount of apps on my phone to a minimum (which ticks me off, since I'd like to use my iPhone as it is intended to be used)

* Turned of all music synching, although I do use Apple Music now, as well as iTunes Match. And yes, every day I delete the Music that was streamed in "Manage Storage: Music"

I am really getting sick of having to restore my phone every month just so it will be usable. Any help would be appreciated.

BTW, I have a 16GB iPhone 5s and a MacBook Pro that's about 2 years old (along with a couple of iMacs and an MacBook Air).