The original idea I would be suggesting is more along the lines of this (I can explain a bit more in depth now that I'm not on mobile)
I would rip up the whole wooden floor as you have it now, and excavate the soil underneath to a reasonable level (maybe 200mm down), then build a wood frame as per the above picture, and overlay the frame with a plywood substrate which you can then put whatever flooring you want on top of.
It may be more expensive than the rubble and concrete fill aspect, but it would put considerably less load on the external walls (if anything it actually adds an additional level of structure support to you external walls)