Yeh... as above. It'd be easier to make up the difference in level with insulation and screed. And... if you want a quick process, then you can look into a quick drying screed.
Normal building practice is to apply a membrane onto the slab before applyign the insulation and then the screed. An additional membrane is usually required between the insulation and screed too. As it's already exposed, you may as well install one.
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