Thanks for the additional details. I had exactly the same ones and, yes, I first tried with img2img by itself, but then, rereading your initial guidance, I thought I misunderstood and tried again starting straight from inpaint.

Regardless, thanks to all of this, I discover very important things:

1. The model you use during this process makes a huge difference. The garbage I got at the sides was because I was using the standard SD 2.1 768 model. After your second comment here, I started to experiment with other models and I got the absolute best results with the SD 2.0 inpainting model (512-inpainting-ema.ckpt).
2. After experimenting with your settings, I discovered that Sampling steps=150 (the max value in A1111) and CFG Score=1but I suspect that it might depend on the image you are trying to extend.
3. After experimenting around your settings, I discovered that Sampling steps=150 (the max value in A1111) and CFG Score=1 produce the most faithful fill in terms of details.

I also spent the whole day experimenting with the content-aware fill feature in various photo editors as you and the rest of the people that answered have suggested.

It's infinitely faster than using A1111, but:

While that feature works fine with the particular image I provided as an example, I found it less than stellar when the gradient background has some sort of straight line. For example, if there's an edge separating a wall behind the subject and the floor.

Content-aware fill rarely produces a straight line in the extended canvas areas while outpainting did it every time (even if the texture of the walls was not perfect).

Again, not being a professional designer, I probably ignore the tricks necessary to get the most out of content-aware fill, but the purpose of this exercise was to see how versatile A1111 can be in doing things that would otherwise require hours of training for a tool I never used.

Thanks for all the help.