Perform an 'Outpaint' directly through the Inpaint tab, you simply need to choose the new size and paint the edges of the image. When the edges are not painted, those stretched lines remain, but painting the edges solves this problem, and the image is generated seamlessly.
I used LCM LoRa, which works really well in Inpaint, using the Samplers: DPM++ 2S, Karras, and Euler A. For this to work, use 8 steps, and a CFG Scale of 1,2.
Don't forget to Select "Resize and Fill" in the resize mode. A prompt is not necessary, but you can use it to aid in the context of the image.
Always use a Negative Prompt to improve the quality of the generated image.
For more complex images, use ControlNet Inpaint along with an Inpainting Model. ControlNet assists in the overall context of the image, and you'll get better results.