“I have to remove the old instance, make a copy then weld all the center X verts which is tedious.”

Just remove the instance then apply the Symmetry modifier to the top of the stack. In the subobject select mirror then move the axis so that the mirror instance lines up with the original. No vertex welding needed until you’re finished with your creation and collapse the stack.

“Again, another something I really never experimented on. I've always used turbosmooth - not sure why - the MeshSmooth modifier definitely has more options.”

Give the HSDS modifier a shot too. See if you like it. Put your smoothing modifier of choice above the symmetry modifier as I show in the illustration.