Alrighty, it's excavation time. To prepare for the dig I made the dig plan below:
The large black square is the existing rainwater tank slab and all elevations are relative to it (i.e. it is the zero datum). The excavation elevations are therefore all negative and can be seen in large font. The smaller, positive numbers are all dimensions. As you can see, it's a pretty complicated excavation.
I also prepared a 3D representation to get a feel for what it should look like post-excavation. With tiered footings on three sides, a pool with various ledges, and the rear retaining wall, it looks more like a poorly executed game of Tetris than an excavation:
The blue area is where the pool will sit, the purple blocks are the retaining wall footings, and the yellow is the pool deck. The pink areas will be filled, so I'll try to keep the existing ground as high as possible there.
Here's the current site from roughly the same angle, albeit from a lower elevation:
For someone that has never dug a large hole let alone sat in an excavator before, this is definitely going to be a challenge.