I think the idea of rammed earth tires is probably the best overall solution ceainly from an environmental perspective
I think you said at the start that rammed tyres were out due to leaching etc. I have *never* seen any data on this issue - but I'd like to as I intuitively feel there has to be outgassing or something from tyres.
FWIW, I'm building a 7m x 5m sb (looking to plaster in a few weeks). The foundations are rammed tyres with railway sleepers on top. There is a LOT of work in ramming those tyres...don't underestimate that. If I were to do again, I'd be looking around for alternatives.
Donald