A tilemap should be used for things that tile.
Remember that it's one atlus of objects in one texture, that usually covers the entire layout.
So you save cpu, and gpu by having one static thing that takes up a set amount of ram.
Multiple different Tilemap objects lower those ram savings.
Multiple objects work the same way.
A layout full of instances is not an issue, unless they are moving.
Also it looks like you have a lot of wasted space. Tilemaps usually have objects close together as in a minimum of 1 pixel apart to maximise ram saving.
You usually have to do that manually.