: If this option isn't in the game, then a good way to mirror symmetry would be
: to have a flat ground and put objects on it (Base, walls, tunnels,
: whatever you want) and then weld it all together, duplicate it, drag it
: off to the side and then unweld everything. Then drag the flat floor up
: (so that it is laying on top of all of the stuff you built) and then weld
: everything to that and then flip the flat floor over. Sure, all of your
: things will be upside down now, but it shouldn't be too hard to flip all
: of your objects over if you need to using the "90 degree
: rotation" option. Beats having to build the whole base all over again.

I was just going to say something like that, but you have a bunch of unnecessary steps there. You should be fine grouping everything, welding everything to the center piece, copying the group, rotating it 180 degrees through the Z axis (spin around vertically), placing it opposite the first group, unwelding everything in the second group, and flipping each piece 180 degrees vertically again. With the new rotation options, it should be just a few presses of buttons and sticks per piece and (twice) for the group and a move of the group one time.