Hi there Pirotess,
I understand that you'd like to make more space available on your iPhone without having to erase everything off of the device.
To start off I will be honest, I'm not exactly sure why you're encountering this problem, but would love to try and help you out. So I do apoligize in advance if some of this information may be incorrect, however I have tried to make sure that the information is as accurate as possible.
From what you've explained to us, it sounds like you're having a problem where the amount of available storage you have on your iOS device is changing every now and then. One moment it might say you have 976 bytes left, then it goes back to being 0, then 320 bytes, and then finally, it goes back to being 0 bytes. As far as what may be causing this, I'm not quite sure. I'm assuming that this might be a glitch with the software, but I could be wrong about that. Have you tried force restarting your device? If you haven't, I'd try doing that now, then checking back to see if you're still having the problem. If you don't know how to force restart your iOS device, please visit this knowledgebase support article from Apple: Restart your iPhone, iPad, or iPod touch - Apple Support.
I understand that you also don't want to install an app to make more space on your iPhone. I've heard of several apps that claim to do the same thing, so I do know what you're talking about. I think this is optional to you. I'm unaware if whether or not they actually do work. Though, if you'd like, you can test it out on your end by having the app to "clean" or make more room on your device, then checking under the Settings app to see how much space is available and seeing if the app did help.
You also said that if you look under Storage & iCloud Usage, it's showing that you've used 11.6 GB on your device and that you have 0 bytes available left on your iOS device. I think some stock apps like the Notes app, and News app on your iPhone might be why your iPhone is showing that you've used 11.6 GB on your iOS device, even if you've tried removing everything like apps you might've installed.
Additionally, here is some more information on making more space on your iOS device. You can find where I got the same steps from this knowledgebase support article from Apple: If you need more space for an iOS update - Apple Support.
In the Usage Settings menu, you can see how much space your apps use and delete the ones you don't need. You can always re-download your purchased apps or other content later. Follow these steps:
- Tap Settings > General > Storage & iCloud Usage > Manage Storage.
- Select any app to see how much space it uses, and tap Delete App to remove it. Some apps, such as Music and Videos, let you delete parts of their documents and data.
You can also reduce the size of your Photo library by turning on iCloud Photo Library and Optimizing your device storage. Learn more about app storage space and content.
I hope this helps you.