I'm progressing through my initial timberframe design from the walls in, ground up, making my way toward the roof, at which point I'll use wind/static load estimates to begin dumping in design numbers and actually engineering the timbers & joints (vs. WAGing it as I go along) back on down. The walls are pretty much self-designing seeing as the various window & door penetrations drive most of the frame geometry, but the floor & foundation seems a bit more open to different tactics.

So the question is how to proceed with the (ground) floor & foundation design given the following;
-Hillside construction
-Somewhat below ground level, daylight basement-ish format (say, 6ft below at the deep end, but about 18" above grade at the shallow end across 40ft)
-Texas Hill Country soil; I would think rocky & limey with a shallow soil layer (probably blown away by the excavation)

This question is two parts, with (I think) two possible answers to each;
Foundation; does it make more sense to try building a retaining wall & using that as the exterior wall & foundation for the structure against the hill, or instead building a standalone wall & constructing the foundation inside it, then backfilling the gap between them with gravel/drain tube? My thought is the latter is more likely to be drier (interior is bedrooms/bathrooms), but much more expensive for what may be diminishing returns vs. some other more effective technique.

Floor; given how dug-in I am, a slab or solid masonry perimeter wall seems a requirement to restrain the soil/backfill and would suggest a slab floor as is done with basements, but seeing how the down-hill side is 'open' or above grade level, could the interior of the foundation 'bucket' be a post/pier scheme with a crawl space rather than a solid concrete 'bathtub?' Would the pro's of an elevated floor outweigh the less-efficient use of excavation, assuming the poured floor would be a good deal shorter than a suspended timber scheme for a given interior height?

Thank you for your thoughts,