is there any way to get non Steinberg synth presets to show up in the Media Rack?
The Media Bay only works on vstpresets, which is Steinbergs format vor VST(3)-plugins. Most, if not all, synth plugins nowadays use their own internal preset handling (because that way they are DAW- and platform agnostic) and Cubase has no way of knowing about these presets.
If .vstpreset files (the official VST3 preset format) aren’t provided by the developer, then you have to right click the top of the VST plug-in control panel and save the presets manually.
It’s a bit time consuming, but at least you can add proper MediaBay tagging from the same window if you want.