I think it's best to make a separate background layer where you'd use a Tiled Background object.
This is an image, most of the time a tile, like the background tile in your Tilemap (or a bigger image is also possible of course), that you can scale all you want and it wil automatically match without distorting the image itself.
Make a layer for the Tilemap above the background layer and use your Tilemap for the ground (collision) and interaction.
You can put another Tilemap on top (in a new layer) for decor purposes, overlapping or whatever you want. Not only is this way faster and flexible if you want to adjust something, like you want to now, it's also better for the performance as the Tiled Background is very efficient.