I have watched a variety of videos on YouTube about this topic.
One guy said that his experience using Hydrogen Peroxide was that within a few months, the white Lego goes yellow again (but WORSE!). He therefore advised against doing this. He might have mixed it too strongly though. He may have also not put it out in the sun. I might do some experiments of my own.
I also found this article last week with a variety of tips to clean old Lego.
Here are a few videos I have also watched:
Side note: Buying and re-selling used Lego apparently is a thing: