I tried using the Export > Audio Mixdown option, but it seems it does not record external hardware instruments. Without having to make a separate Wav of each hardware instrument track, is there a super simple way to simply stereo record what is being played by the midi tracks?
Have you tried exporting in real time? (Tick the Export In Real Time Box)
As far as I know, no, there’s no easy way (see manual for details). But from your post it’s not quite clear how you are doing this. For example, are your external instrument outputs already connected to Cubase?
The audio outputs of your external MIDI instruments must also be connected to Cubase Pro inputs for this to work.
See here: Setting up External Instruments
Or you could route your complete mix (whereever it is, presumably in your external mixer) back to a stereo input of your audio device and just record to a empty audio-track within Cubase in realtime.