With ffmpeg how do I imply spatial position when mixing mono to stereo
This is something that we can't really give an absolute answer because we don't have your inputs and what is "best" will be fairly subjective.

You could just try using the pan filter with a bigger gain factor for one side. Something like:

pan=stereo|c0=0.8*c0|0.2*c1=c0


Depending on your inputs adding adelay may make it sound less flat:

pan=stereo|c0=0.8*c0|c1=0.2*c0,adelay=0|10


You can use stereotools filter if you prefer as it has many options (I completely forgot about this filter...can't keep up).