Hi Jef,
perhaps something on top of what Michael suggested.
Mix all tracks into a 7.1.4 bus. You can position each individual source track to any 3D-position. Can also put a 3D-reverb onto this bus. Or none, if kind of “pure instruments” is the goal.
The bus feeds into another 7.1.4 bus. This can gets two instances of In-One Binaural Plugin (New Audio Technology. (Reason for two is , that one can handle only up to 7.1. One of the next versions will not need such workaround).
One instance gets e.g. the Bottom 7.1, and the 2nd instance the Top 4.0. In each of the instances, you can (again have to) adjust the 3D-XYZ-position. Can also apply certain presets to make it sound like in a studio.
Just bought this plugin (< $100) after having tested about half a dozen of Binaural Plugins (DearVR, Ambeo, Dolby Atmos Production Suite, Steinberg Immersive, Ambisonics and more).
BTW, I am not any close to be an employee of this company.
So with this approach and plugin, you can go even beyond “5.1 on steroids” as it adds the 3rd dimension of heights. Sure, can do 5.1-on-steroids with it too.
LG, Juergi