This is an excellent image, it’s really well done. The colors are gorgeous.
Back to your question. I usually find the cloning / spot removal tools in Photoshop to be more powerful than Lightroom. Photoshop Content Aware Fill can also work wonders sometimes. I usually use Lightroom spot removal only for dust spots in the sky, anything more complicated than that usually requires the PS tools.
As to the cause of this, it could be something on your sensor (most likely) or a smudge or water on the lens. It’s too large to be a dust spot. A sensor cleaning might be in order. I have also seen large streaks like this in the sky get created by birds during a long exposure.