Heightmaps basically are grids of numbers expressing altitudes. At your present resolution, every pixel defines the altitude of a 1.5 squared meters area. Have you tried raising the resolution of your heightmap to something like 1px = 1sqm (or even higher)?
Also, at 16bits you have a range of 65535 levels of grey to express altituteds. If you set your world to have a maximum height of 65535 meters, this means every tone of grey will be equal to 1 meter. With a 12x12Km extent, I doubt your mountains will be anything wider than 4x4km each at most, which means the maximum altitude will probably be no more than 3000 meters. If you set your world's maximum height at around 3000 meters, your resolution in terms of grey tones would be 20 tones per meter, 5 centimeters per grey tone. Plenty of resolution, I'd say.
Anyway, a visual hint of your problem would help define a proper solution much faster, mind sharing a screenshot of how things "go south" when you scale things down?
I'm currently struggling with a workflow for designing small features that I want to keep it's properties as I scale them down in XYZ.
My current workflow is to design them as small as possible while still getting reasonable look-feedback in 4k. Then I create a master scale parameter, then using scalar arithmetics to proportionally adjust every feature I think will impact scale, as I scale down the master parameter.
Purpose: To design features in a reasonable size in the preview window, and then scale them down to required detail level.
Problem: When I render scale down, with even with quite simple device networks, nothing keeps its look.
Practical Description: I'm working on terrain 12x12km, (8k build res) and I have designed some fancy looking sand dunes. The issue is that they are averaging 200m between dune-tops, and I would like them to average more like 20m. When i Parameter-scale them down, even very simple device-networks fail to produce desired results.
Anyone have thoughts or solutions to this problem? I find the master parameter->arithmetic workflow really unreliable..
Perhaps: Pre-generate a tiling texture, scaled in photoshop? Can any tricks be done with the extents? Render hi-res to library and scale down?