From what I've read online Max raid battles don't have an increased shiny chance. However there seem to be certain people doing Max raids that almost always have a Shiny Pokemon. I'm not sure how they do it. The key is when you're searching for Max raid battles look for a name with the same profile picture to pop up multiple times throughout the day, usually with the same Pokemon for each raid. For example there is a person named Phoebe with the female swimmer profile pic that almost always has a Shiny Pokemon for their Max Raid Battle. It took me a month of shiny hunting to gather a short list of people that almost always have a Shiny Pokemon for their raid