I appologize for being to lazy to look it up, but does your phone provide for an external SD card?
It would be great for you, even though you don't seem to have a ton of personal stuff on your internal memory.
First, that is more than 1GB that can be moved out of the internal memory.
Second, it would allow for easy separation of your personal files.
Since you are on Android 6, I assume that you have a built in storage system setting that allows you to clear the system cache right from the phone storage page (see photo above)?
Just touch the cache area, and a notice saying that you will clear the cache should pop up.
Go ahead and do it, if you haven't already.
That can easily be up to or more than 1GB.
Now, go into your file manager, and look under your normal internal storage at a folder called DCIM.
Insure that 'show hidden files' is active on your file manager.
(Simple Mobile Tools makes an excellent File Manager, btw)
Inside, there should be a file/folder called thumbnails or .thumbnails
( . files are only visable if you have allowed 'show hidden files')
This is where every single thumbnail for each photo, video, avatar, ad, etc. is stored.
I have seen this useless junk file get to over 3GB!
Delete that crap.
Now back out one step to the DCIM folder, and add your own file-
.thumbnails
and save it.
This will prevent more thumbnails from being saved.
Yes, some galleries and video players really need thumbnails, and will start up slow without them.
If that is the case, get a new gallery and a new video player!
Once again, Simple Mobile Tools makes an excellent Gallery (pro version) and X-Player will play virtually any video that you throw at it.
Drawbacks?
Thumbnails allow for the retrieval of photos that have been deleted by accident.
But Simple Gallery Pro has a Recycle Bin, where everything you delete goes before it is permanently deleted.
So, no real drawbacks as far as I can tell.