What kind of screed is made on loose sand with stones?

My floor in the newly purchased old house consists of stones and sand. What do I put on it to get a firm, even floor? Only boarding is to be laid on the surface. So there will be no heavy loads. And I have to carry everything very laboriously up a mountain (50 meters elevation). Therefore, I try not to use unnecessarily much material. I am assuming about 2 cm thickness. Should I use concrete, self-leveling screed, or what is recommended?

Do not use screed, but make a battens for the floorboards - just as it was done in the past.