That is one scenario yes. And I think I have a test case that works. With emby, it seems to be ultra sensitive to the slightest change. E.g. I merged a second mount in with similar root paths but only a handful of new media. Emby chose to see everything in the existing paths as new. I don't know if this is because the modify date on the root folder changed or what. So my thought was, is there a way to freeze that initial metadata in place.
So I ran a test with a fresh emby, fresh vfs cache, and fresh scan. Stopped mount, tarred cache dir, forklifted that to a new machine, fresh emby install again, and initiated scan. In monitoring nic traffic, it seemed to be next to nothing compared to the first machine. So I think this will work for my use case to create a tar after a fresh scan as a baseline.
The larger issue I'm trying to tackle is emby 4.5.4 to 4.6.4 upgrade. For whatever reason this upgrade seems to wipe libraries and thenemby devs said this is normal and a fresh scan is needed. I have a home emby server and one in the cloud, so this may lighten the burden of the upgrade. I guess I just needed a sounding board to work it out in my head. Thank you as always.