Unfortunately, du is kind of it for folder size calculation. I have also looked for better/faster tools to find directory sizes and there really isn't one that I can see. There might be some 3rd party custom binaries that can do it, but then that would involve deploying something to your Macs just to pull an EA value, which doesn't make a lot of sense.
I had hoped that things like Spotlight (mdls or mdfind) could see that info, but it doesn't capture it. You can't rely on ls either. It's not recommended to use it in scripts as the output can be unreliable.
What I might recommend is crafting and deploying a LaunchDaemon and companion script very much like what you have that would run every x hours. Instead of having it work as an EA, take the same data and pipe it into a local file or a plist of some sort. Then have your EA look for that file/plist and scan for the value. This will make your inventory collections much faster since it's just picking up a previously recorded value, and will still keep it relatively current if you have the LaunchDaemon run, say, once every 2 or 3 hours to update that locally stored info. The LD will run independently and automatically and therefore not be tied to machines checking in/running policies from Jamf Pro.