I don’t know if this could help you, but I use a head set with MSFS and Vatsim.
In Windows sound mixer you are able to set the relative volume of both applications.
So I have MSFS ATC disabled and for me this sounds as real as it can get. I have all the sim sounds: aircraft, environment, and the (sometimes busy) chatter of the ATC. There is no interference between these.
To mimic the real life scenario where the sim sounds come from the speakers and the ATC through the head set would mean that I would have to crank up the volume of the speakers quite a lot to overcome the isolation of the head set cups, so that I don’t do. In that scenario you could work with a separate microphone like in the olden days.
Hope this helps a bit.