Greetings gentelmens
Spacing >4 gives good “spherical” shaped plume. Nice, but lack of details (wavelet sim after spacing >4 gives not great looking details).
Spacing <3 gives “that” square shape.
How to beat that problem? 
Spacing isn’t your problem, There is no 1 value to rule them all.
What spacing does is tell the voxels inside the cube how small they should be, Smaller values give more detail but take longer to simulate plus cost more ram.
Reset your other values to default and start again, Seems like you’re introducing some really strong gravity or something along those lines that forces your smoke to come to a halt, Either that or somethings wrong with your bounding box.
I always used comparison videos for different values to understand how it all works, Something like Jeff Lims website FumeFX – Vorticity and Grid Spacing | Jeff Lim
Good luck.