Acidic subsoil treatment for foundation

My own feeling on this is that more detail is needed. This question is not one that can be answered "in principle". That may be why you have had no replies to date.

I have some misgivings about your proposed method of saving on the more resistant concrete. Using only the lean mix concrete as a barrier, how long do you think the lean mix will act as a barrier in that environment? If it IS lean mix, then it will (by definition) have a lower cementitious content. If it is lean mix, I presume it will NOT be reinforced. In that case, surely you will expect it to crack significantly, offering a path for the contaminants straight to the Normal Portland Cement concrete! (I am assuming that the groundwater is also acidic, although you have only quoted results of tests on the soil.)

Remember that you have a contaminant SOURCE, and you have a TARGET (your structure and its foundation). If there is ANY PATHWAY by which the contaminant can reach the target, you need to design for that situation. Eliminate the source completely, or remove the pathway (e.g. by adequately protecting the target), or move the target away.

I would suggest that if your pile cap needs fly ash cement, then your basement slab probably does as well.

If I have misunderstood the problem, perhaps more details will encourage more responses.

I see proper using sulphate resistant cement (fly ash or not) for the whole foundation, piles and walls included.

Spanish code EHE classify some of your pH Strong Attack Qc hence Sulphate resistant cement is enforced.

Given potential incompatibilities, EHE code only allows use of fly ash with CEM I (or pure Portland without any fill). Hence if you want to use fly ash with SR cement, I see a certificate on the compatibility of the fly ash with the cement long term in the presence of the agrressive environment would be mandatory.