I don't overthink it. I just make sure the main topic appears naturally inside the caption instead of stuffing the same words twenty times. If the post is about photography, i mention photography normally while telling the story. Same with the location or the name of the product. People are not the search engines alone, they still read the caption. Once it starts sounding forced, I lose interest immediately. Hashtags can still help some extent but i don't find myself depending on them like before. Better picture, useful caption and genuine interaction still carry a more weight.