(Disclaimer, not reading every page of replies)
To me there's a difference between "overthinking" and "overcomplicating".
I think most brewers underthink their brews. However, I also most brewers overcomplicate their brews. Whether it's stupidly complicated malt bills, or stupidly complicated hop schedules, or stupidly complicated spice additions. Yet they pay no attention to basic fundamentals like removing chlorine or chloramine from their brewing water.
I get very scientific about my beers. I (usually) take and record in my log the following measurements with my beers: grains to 0.1 oz, measurement if strike water down to the fraction of a quart, strike water temp in kettle, strike water temp in MLT, water additions measured to 0.1 gram, mash pH prior to adding acid, mash pH during addition of acid, mash pH after resting for 15 minutes, any mash factors (infusions, decoctions, etc) to fraction of a quart, initial mash temperature, mash temperature halfway through, sparge water temperature, gravity of first runnings, gravity of sparge runnings, pH of grain bed during sparging, preboil volume, preboil gravity, boil time, hops to the gram, hop additions to the second, boiloff rate 50% of way through boil adjusted for thermal expansion, chill time, chill temperature, pitching temperature, pitching rate (calculated), aeration time, post boil volume, original gravity, loss of wort to trub, fermentation temperature, FG, bottled volume, bottled beer temperature, carbonation level desired, priming sugar measured to 0.1 gram.
What this does is tell me when anything is off. And I can tell you with really good accuracy exactly what my mash efficiency will be for a given grainbill. It also enables me to rebrew the same beer every time so the numbers match up perfectly (unless I'm intentional tweaking something) and the beer tastes indentical (again unless I'm intentionally tweaking something), and then if I do tweak something, I can know that whatever changed in the beer was a result of the tweak that I made.
HOWEVER, whenever I'm coming up with a recipe and get to exercise the artistic side of brewing, I ask myself for anything that I'm adding "what is this adding to the beer?" If I can't figure out what adding something is going to do, then I'm not going to add it. And adding too many things leaves you with an overcomplicated muddled mess.