I had really good results using the Sylvania headlight restoration kit. There were quite a few reviews online and youtube etc that gave it good marks. Essentially all of them give you 1000/2000 grit sandpaper and you polish off the old yellowed plastic. The trick seems to be in the cleaning agent and coating that the Sylvania kit has. There is a pre-wash that they say softens the plastic before you commence the sanding/polishing steps and then the finishing or sealant that you use to coat and protect when you completed seems to be holding up well. Here is a link to a test online and you can check it out.
https://www.popularmechanics.com/cars/how-to/g1470/which-headlight-restorer-works-best/