Just a heads up .....
I don’t really know much about stem walls with backfill etc., but I do recall (but very sketchy on the details) that several years ago a guy here on GJ had a similar site where he built a garage on a slope. To get the front of the garage level with his driveway, he had to build a 6-7 foot or so high block foundation wall at the rear, with foundation side walls tapering up to driveway elevation …. not too unlike your situation.
He had the block walls built, then back-filled and compacted the inside area of the foundation, poured the slab and built a very attractive garage. Then, somewhere in this process and before it was all done, the rear foundation wall buckled out, floor tilted, and it all went to hell.
Big ******* contest with builder, city, etc.,etc. All went on for months. I wish I had kept a link to the thread …. but it should be in the Garage Gallery section somewhere, IIRC.
Bottom line lesson to be learned …. If you go this route, make sure the foundation, foundation deadmen, and backfilling steps are designed and executed properly or you could have a disaster!!