iPhone 6 low on storage and becoming non-functional
I am assuming here that you have synced your phone to a Mac or PC and continue to do so? If so and you have the options in iTunes checked to sync photos (on a Mac it would be photos.app and on Windows it is usually a folder in your Pictures folder) you should be able to verify that they are on your computer.

Once you have found your missing photos and verified all you need is safely on your computer I would do a wipe and restore. That cleans out temporary/cached files and files left over from deleted apps and the like. To be safe I would make another backup of your phone on your local computer. And I would password protect the backup so it preserves any passwords saved on your phone. Once that is done...

On the phone it is in Settings > General > Reset > Erase All Content And Settings.

Erase the phone and then restore it from the backup you just made. That, in itself, may clear enough space on your phone to help the issue. If not you may have to remove some apps, music or photos to clear more space.