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The soils that the crushed stone is on - are they virgin, or is that fill? I ask because the rest of the backyard looks somewhat freshly filled and/or graded.
If so, with 7-8" of uncompacted stone already down, I'm worried about the long term durability of the base prep.
For the time being, I'd stop adding any stone (or sand or anything), and try to rent a reversible plate compactor (something with around 10,000 pounds of force (I don't recall the equivalent kilonewtons)) to pack that stone down. At 7-8", the standard 5 horse Honda-driven compactor won't get the job done. Just too much material to compact.
But I would think pouring the slab as step support would be a good option, especially considering your geography.
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