I don't have an answer for your specific import problem, but since you're looking for a workflow for large terrains, I'll offer the following suggestion:
Don't load terrain data from a 3D file format. Instead, assuming your terrain uses a regular rectangular grid, convert the terrain data to a height map outside of Godot and store the terrain as a 16bit greyscale image (or, if you don't need high precision for the height at each grid point, 8 bits may suffice).
(converting a grid-like 3D mesh into a heightmap is fairly simple; you can probably do it inside blender by baking with an orthogonal camera looking down on the terrain and a clever shader. Or if that fails, you can always export the mesh as a OBJ file and write a small Python/GdScript tool that reads it and translates it into a heightmap image)
Now you'll only have to load an image file, which is much smaller and makes for faster load times. Consider that storing a mesh as a collection of vertices uses at least 12 bytes per vertex (3 floats, each take 4 bytes). And that's just for the raw vertex data - that doesn't include the definition of which vertices make up the triangles, or uv coordinates. Compare this to a heightmap image, where each pixel represents a vertex and only takes up 1 or 2 bytes, and no uvs and triangles need to be defined because that's implicit in the position of a pixel in the image.
You can then create a a normal, rectangular plane mesh that's subdivided the correct number of times in Godot and use that as your terrain mesh. Then use a custom vertex shader that reads the height coordinate from your heightmap texture.
Have a look at the "Your first spatial shader" Godot Tutorial at https://docs.godotengine.org/de/stable/tutorials/shading/your_first_shader/your_first_spatial_shader.html for details.