Hot poured bitumen is a traditional material, used as a damp proof membrane for centuries.
We knocked through our kitchen to an existing extension, where the floor levels were out by 2" down to 1/2".
A skilled bitumen specialist will give you a perfect finish to lay a screed or floor covering.
It's priced by m3, area x depth for material and m2 for finishing.
Iirc around £30/m2 for our job.