How do you transplant a very large Gardenia bush?

https://www.gardeningknowhow.com/ornamental/flowers/gardenia/transplanting-gardenia-plants-planting-gardenia-somewhere-new.htm

The root ball should be the same diameter as the canopy of the shrub. Dig wide, but dig shallow (12"). The planting hole should be the same size as the root ball. Keep well watered until established. Consider pruning back by 1/3 after plant has bloomed next spring.

Fall's the best time. About a week before the transplant, prune back the branches by one-quarter to one-third. This helps the plant focus on its roots. And chose a cool day for the transfer if possible which is good for you and the plant.

Prepare the holes for the gardenias. The location should be a place with rich soil and light shade. Augment the soil as needed to ensure the ph of the soil is between 5.0 and 6.0.

Dig a rootball as big as possible around the plant. The more soil around the roots the better. Transport the gardenia and its rootball to the new spot and plant immediately.

The size of the hole depends on the plant you are moving. Gardenias are generally a shallow-root plant, but it's hard to say that's 6 inches or ten. The new hole should be about as wide as the plant you're moving and about as deep. To dig out the plant, explore a bit by using your spade at the shrub's perimeter. If you hit roots, go out wider until you hit soil and no or few roots.