If I keep (naafil) supererogatory fasts with the intention of losing weight, will I gain rewards?

The main thing in worship is to do it with one hundred percent intention for Allah. In addition, unless the intention related to the aspect of worship is reduced to a simple level, other purposes can also be mixed with the intention of worship. This interference does not abolish worship, biiznillah. There is no harm in making a pilgrimage for commercial purposes besides the intention of worship. There is no harm in fasting to break the lust. The same is true for weight loss. As long as the intention of worship is not considered as less important than the other.