Is it normal for LaunchD to write over 600 megs of data to /private/var/db/dyld/dyld_shared_cache_x86_64?
Yes, it is normal.

On macOS the system stores a combination of the most recent (i.e. highest version numbers) dynamic libraries (frameworks, etc.) in this cache file in order to speed up the launching of processes later on.

When switching from another OS (like Linux) it is normal that you have to get reacquainted with how the system works. As you'll learn, there's a quite big overlap conceptually between macOS and Linux. Most everything in macOS has a Linux counterpart and vice versa.

In this case the dyld_shared_cache is somewhat similar to the /etc/ld.so.cache file on Linux. On Linux this cache contains the most recent shared libraries, which are essentially the same as dynamic libraries on macOS.