Dig till you hit rock and put the pipe down in screenings. Not to keep pushing you to closed loop, but the ground conditions you mention seem ideal.
Happy to perhaps have this back on the table as one potential option. The real issue I envisoned (beyond bringing in decent fill for the intial backfill) is that 48" is the area's rated below frost line. When I put in the well water line I had big equipment and it was working to get me that deep. They were able to push the basement 1-1/2'deeper by opening up a large space and finding some softer areas and then spreading out. I think I could still get away with this if I use the slinky approach in a single wider trench. Nothing is impossible, just time and money. It seems that I am at a point where I need to get a water analysis and also see if I can expand my local contractor search a bit and get a range on estimates for closed loop option, since I can't do that one myself! I know there is no such thing as one correct answer, this discussion is very helpful to me!