So maybe this is a larger topic that should go back to how the Android OS is designed, but even with a Galaxy S5 with 16GB Ram, system updates are next to impossible as I have to dump various apps to keep available memory under 1GB.
Even using AppManager III (Mid 2016), and obtaining an 64GB Samsung MicroSD card, having move ALL apps that can be moved to the SD card, this problem persists. I regularly offload and purge photos and videos, so media taking up space (on the main device storage) is not the problem. Does Android reserve large blocks of memory that are simply un-recoverable without a system wipe?
Where does rooting a phone come into play? I'd rather get on with my life than defeat the purpose of conveniences, but that's current tech.
This topic appears to circulate across many forums, yet with little straightforward resolution.
I had over a gig of SMS and MMS messages. I just backed them up to Google Drive...and have not restored them yet. Doing a full system restore did not make a huge difference (maybe 500 megs), but deleting the SMS, MMS, and not having much private content (if you use that feature, items are moved from the SD card to internal memory) made the largest difference to me.
I've noticed as of late that I have to pick and choose apps wisely, otherwise I'm close to the 16GB max. I'm still able to have 25+ apps downloaded and 3GB available, but I don't keep a lot of videos, pictures or music on my phone.
Under Settings>Storage share a couple of screen shots to see what all you have on there.
Under settings>storage and system memory I'm showing just shy of 6GB being used, is your astronomical?