Levelling compound for screed, raw ceiling is uneven

Hello everyone,
We have now been in contact with 2 screed layers.
It is about our renovation and the upcoming screed.
Our subfloor is very uneven (see pictures)
One screed layer would recommend that we mix "a little" concrete ourselves and thus level the holes.
The other screed layer says that is complete nonsense.
He would level the floor completely so that you have a proper structure.

Can someone with expertise explain the structure to me or how they would handle it?
Underfloor heating will be installed.
Best regards

Can someone with expertise explain the structure to me?

What are you currently doing in the house and what is planned? The structure depends on that. So specifically: What goes on the raw floor? What insulation is planned? What build-up height do you have? What ideas does the planner of the measures have? ...