This is what you need, but it’s not a free-back-and-forth-all-day-long process.
You want to be very intentional and parsimonious about how many times you go back and forth because there’s a LOT of room for error.
When you drop a Blender file in, Unity has Blender run a Python export script, then imports the resulting temporary FBX file, which you don’t ever see in the filesystem.
When you drop that (or any model file) into your scene, it makes a GameObject, gives it a MeshFilter and MeshRenderer. The MeshFilter is pointing at the Blender/FBX file.
If you “Probuilderize” that object, it breaks all the links to the original Blender/FBX file mesh and creates a Probuilder mesh in scene and connects everything back up in that one object, giving it everything it needs to be edited in Probuilder.
You now MUST save that scene or prefab because that’s where your modified mesh is now.
Once you do that there is NO connection back to the Blender file. You could even delete the Blender file at that point. This obviously applies to any other model that your mesh originally came from: it is out of the picture now because the data is in Probuilder format in your scene / prefab.
If you wanted to bring changes after that point back to Blender, you need to export it with something like ExportOBJ and then import that into Blender.