The current “Classic” uses the same updated under the hood tech that the retail client uses. That’s why the blades of grass swinging in Classic happens - it’s using modern tech even though it’s using old game files.
The nVidia 300 series is from nVidia’s Tesla architecture and is not supported by Metal. As WoW no longer uses OpenGL and uses Metal exclusively in OS X, you need a more recent machine on the OS X side. You might get by at minimum settings on the Windows side using that platform’s WoW/Classic client(s), but outside of that I’m afraid you need to update to a newer Mac.