Mesh Leveling
So, your bed is not flat. The first layer can not have a consistent height. What do you do?
For years you lived with it. That’s why rafts were invented, so the wavy first layer was part of the raft you threw away. Rafts are horrible. The surface against the raft looks bad and they are hard to get off. Do not use them.
Mesh leveling was invented. You figure out where the bed is at a bunch of points and imagine the bed flowed from point to point. You get a mesh that is close the the shape of the bed. More points and the closer the mesh is to the bed, and the longer it takes to get the points.
Now, as you print the first layer as the head moves around move it up/down to keep it a constant distance from the bed. A perfect first layer. As you move the head to the next layer move the head up/down less and after a few layers you have a flat layer and each undulation has been split and only part of it is on the early layers.