I thought red at first, and if it were only slightly different parameters I'd still choose it, but 6? That's too far back to be trapped in a child's body and environment. At least going back to a more plausible age for a grown up's mindset like teenage years would be a bit easier to deal with and to lay low, it'd be strange how much more mature and less reckless and slightly boring of a teenager you had suddenly become, but at least it wouldn't be like international news. At 6, life is going to drastically changed by your seemingly impossible linguistic skills alone, child development experts would want to study you, you'd now be a prodigy, not necessarily a bad thing but unless that specifically was the path you'd always wanted but never achieved, you'd now be pretty well set down that road and all that comes with it. The relationship with your parents would be so different and they'd be robbed of your childhood and suddenly have this adult they'd never met before to deal with after barely getting any time to get to know their own child. It'd be so frustrating too, no one would let you drive and you couldn't drink, or fuck. You'd hopefully be able to get yourself some more autonomy than your average 6 year old if you revealed all your cards right away because it'd become immediately clear that fisher price toys and curfews and first grade weren't appropriate for you, but even so your adulthood, already well underway by this point would be drastically curtailed for something going on a decade. Maybe you'd decide to play like in a movie and adopt secrecy so your parents and peers don't know how smart you are, but that'd frankly be way worse and so exhausting and lonely and alienating.

If this was, maybe start again at 14, or better yet 16, I'd take that red pill no problem. It's most of the benefits of the blank slate try again with benefit of hindsight premise, but skipping over the parts that would be simply intolerable for an adult. At 16 you're a 'young adult' getting to relive some of the things you miss about being a child but with many of the benefits of being an adult and biologically you're pretty much over the worst of it, if you really hate the social restrictions imposed upon you by being not technically an 'adult' you're only 2 years away from fixing that, not over a decade, and when you get there you'll be in way better control of the trajectory of adulthood. Most of the really decisive things about adulthood that trace back to childhood happen around this time as well so it's where you'd get the most bang for your buck. You can take a very meandering path up until that point and still change direction but this is where decisions start to become more binding and long lasting so it's really the point where most people, if you asked them, would probably begin making tweaks if they could. I reckon the details about one's current life that most people want changed wouldn't have any meaningful correlation to things they did when they were 6, it'd be things like their career, or relationships they've had or wish they'd had, it'd be academic ability or a better body not ravaged by years bad lifestyle choices pretty much all of that is something you could very impactfully change at 16 without the need to learn to read all over again.