I would place extremely large boulders to conceal the access to that crop, and stagger randomly placed smaller rocks to gradually lead back toward your paving. Keep the shape and flow natural looking with curves and backfill with river pebble a dug out trench that runs although it like a dried up river bed. This will be useful in wet weather, it will actually help direct the overflow of water that your property must suffer from. Possibly remove your storm water capping and repace with heavy duty steel mesh grate covered with river pebble. The sound of water rushing over the pebble and draining away will provide acoustics of a rainforest or running stream at the least. Use tall plantings to conceal this void space with tall palms or giant birds of paradise, strelitzias will have the look of banana leaves, and smaller strelizias will complement them. Plant densely so that it prevents access for little ones. Then use Clever lighting to focus a green oasis. Your limitation could possibly be an advantage in that the solution has to be unique. You may end up with a stunning water feature and private escape.

This was my solution to my sloping front garden....just bought lots of small ...as in tiny pots...of various conifers...and over the years this was the result

It’s a sliding door to go outside, so the lounge can go in the front of the wall and sliding door fixed panel. Bring it away from the wall. Or two accent chairs with a coffee table. What is around the corner that doesn’t show in photos. If there is nothing that the lounge is facing, then turn the lounge around so it faces the dining area.

I feel for you too. I suppose the cost and engineering to lower the garage section now would be prohibitive? I think it can be done, I saw a renovation happening a few suburbs from us where a garage was inserted under the existing house, it was fascinating to watch. I don't now recall the details but when we were looking for land we came across information about garages being allowed forward of the building line if the slope exceeds 'something'. Of course rules vary from council to council, but I hope Siriuskey's suggestion will be an option for you. How troublesome that slope is might depend on the vehicles you use. We have had different vehicles some which struggled over particular locations and some which didn't, even public car parks which you'd think are designed to cater for all. You shouldn't have to choose your car to suit your driveway. You said it's a "custom house design" so was there an architect involved?