ᐅ Door threshold removed—how can I create a level, flush surface?

Hello everyone,

Today we removed a door in our house and took out the threshold at the same time, with the plan to make it flush with the floor in the future. On both sides of the threshold, wooden floorboards are installed, and the threshold itself was also made of wood and creaked quite a bit, which is another reason for removing it.

Now my question: Is there a way to install something solid there (like screed or similar)? Currently, the base consists of a mix of stones and wood.
I have attached a photo showing the current condition.

If you zoom in even more with your photo, you can see even less ;-)

Try taking a wider shot with your photo. As for the site, I would remove all the old material and then actually install a screed.

Here is a picture taken from further away

Hello questioner.
To get straight to the point:
You have a subfloor made of wooden floorboards.
Wet screeds (except for large areas with stone-wood screed) have no place here, to be clear.
The area is leveled by screwing together wooden board sections of the same height or (better) with a precisely cut wooden chipboard or OSB board (possibly doubled).
Pre-drill and fasten to the existing subfloor with Spax screws (full thread!!).
Regards, KlaRa