I have been working on this problem for many months myself. It is apparently a problem with the BIOS. They do not want to fix it so we are stuck with the problem. I have tried all the supposed fixes too and none worked any longer than a reboot. You have three choices. Either make the tablet all Android or all Windows making more storage space. I choose to change my android partition to 20GB and do a clean install of the latest Windows 1909. With this set up I am able to load any apps and Windows programs I want. I use storage on my home network for files or you can use cloud storage for file. One last suggestion. I made an image of the tablet using Acronis once I had it all set up because I also have been having problems with the Android partition storage shrinking with no apparent reason or fix. This way I can restore it if the storage mysteriously gets filled. I also bought a very small USB drive but do not like having it stick out too so I do not use it much.
Link how to load newest Windows
Link on how to reload Android and Windows
Note: When loading Android the files must be on the Root directory
Link to make Android only or change the Android partition size