RE scaling the height maps.
Be careful. You want to try and maintain the correct ratio.
If you downloaded a 30m DTM you get 30 meters of terrain in 1 pixel of height map.
So, to make that 1 to 1 you would have to multiply the size of the height map 30,000%.
Doing this will obviously create artifacts and terracing.
If you download a 5m DTM then, the stretch required will be far less.
If you managed to find a 1m DTM, youd have no stretching…