Most synths with large libraries can handle libraries from a different drive…
But it depends on the plugin how this is handled.
You can move the Steinberg libraries now, to free some space on C:\
For example, the quickest way to free up some of your C: drive is to first move the folder:
C:\Program Data\Steinberg\Content
to another drive, e.g.:
D:\Steinberg\Content
Then, using the menu items that Link Shell Extension added to Windows Explorer, right-click on the Content folder in its new location on the D: drive, and select Pick Link Source.
Then, back inside the *C:\Program Data\Steinberg* folder (the original location of the Content folder), right-click on an empty space and select Drop As… Symbolic Link.