It might be best to transplant in the Fall when temperatures cool and then also it won't matter about the leaves,because they will come off naturally.
But if it needs to be moved now,yes,I'd cut it back about 50% or to to about 3-6 feet tall.Pruning will promote growth.Water the plant well. Brady

Not pruning back would absolutely be best for the shrub. Just as you said, the more top-growth you are able to keep, the better equipped your plant is to regrow roots. However, cutting it back may make working with the plant much easier and may not necessarily hurt your shrub too much.

I agree with the recommendation to wait until later (fall, or maybe even early spring) to make the move.

I believe you may find a lot of useful information at the link below.

Here is a link that might be useful: Planting a Tree or Shrub - particularly section 11

If you are going to move it in the fall, I would sever some of the roots you will not be able to save, NOW, by circling the tree with a spade pushed all the way down. This will cut roots you will have to leave behind, and cause the tree to grow new feeder roots within the soil you will be able to move with the tree. I also would not cut back the tree until the day of the move. Al

If you can put up with it where it is for 6-9 more months the best option would be to layer ie. take a long lower branch and root it in the ground next to the original bush without severing it from the parent plant until it starts to grow independently. Elderberry does not root easily from semi-hardwood cuttings, the only other way I have heard of that it can be propagated. I believe it can be grown from seed but if it is a special cultivar it would not come true and would in any case take an age. I'm afraid your chances of digging it up and transplanting are not good. Good luck - Ian.