I presume you mean “configured as an external instrument” (which can be handled somewhat like a VSTi)
The synth just has 1 stereo output port
That’s your limitation right there. You have to render tracks with external instruments or effects in realtime due to the laws of physics, and no software, open source or not, can get around that part of the task (unless perhaps open sorcery). As you have only one stereo output, you have to render each track seperately (i.e. solo each track and render it in realtime).
Have a look at using a macro to render one track at a time, and assign that macro to a key command. A good starting point would be one of the rendering macro presets (Edit > KeyCommands > Show Macros > Presets > Rendering Macros and have a look at the “Render VSTi” macro).
If you can get the process down to a series of repetitive key commands in Cubase (i.e. using macros), then any keyboard automation tool (Windows users have the open source AutoHotkey) should help running those macros, however it would still take a very long time, in your example 40 x 10 = 400 minutes or a little under 7 hours (not 60) – overnight, in other words.