I’ve been happy with the “ReviewMeta” chrome extension, which evaluates the product reviews on any amazon site, and displays a score indicating how likely it is the ratings have been manipulated. It shows examples of flagged reviews which can be pretty funny.
Chrome lets you limit extension permissions by domain, so I don’t worry about it snooping on the rest of my browsing.
But you’re right about brands. When it comes to certain products like phone chargers, the manipulation is so rampant I stick with the same supplier.