Turn off colliders in the layer.
6k tiles in one axis will be slow to generate, so you probably will want to split it up into multiple smaller tile maps, and only generate the visible ones at a time. Similarly, you can do the same with the tile map, by SetTile(x,y) one partition at a time and call Build incrementally over a few frames.
If that is too slow for you, you can still do some additional stuff with a static sprite batcher, but I think it'll be possible to amortise the cost over a few frames.