It sounds like you may be over complicating it in your head a bit. A directly attached USB drive, unless the drive itself is the bottleneck, should have over double the bandwidth of the 4K’s 100mb ethernet connection. A USB connected drive will always be mounted at a system level so using fstab to configure it only changes where it is mounted, not the performance.
As for your library that can continue as you currently have it as long as you are smart about how you do the switch. If you currently have your library entries pointed to a system mount then you should be able to just modify that so the new directly attached drive is mounted such that the file paths would still be the same. For example if your pointed such that Kodi sees /mnt/nas/movies/some_movie/some_movie.mkv then you will need to make it so that exact file path is still valid so “nas” would need to stay as the mount folder and the file naming and organization would have to be emulated as well.
Alternatively you can use path substitution to redirect from the old location to the new one. With this method you would be telling Kodi that (using the example file path above) when it sees /mnt/nas/ redirect to /media/usb/ and it will manipulate the file paths on the fly in the background. I wrote a how-to about how that works that can be found here…
The main thing you absolutely do not want to do here is edit your existing sources in Kodi. This will lead to duplication of items in your library, dead links, and is a pain to fix without just deleting your entire library and starting over.