I'm not trying to argue, just provide some clarity for new brewers. Sours are becoming increasingly popular, so I'm sure lots of people will be around trying to brew them. May as well get some info to them about how to best accomplish that. Otherwise, you get misinformation flowing like "to get a sour, you leave the grains out in a bowl."
The grain bill for something like a traditional flander style sour is pretty simple. Pilsner, munich/vienna, corn, a little special b or other dark malt. Lots of unmalted wheat is common in some modern brews, but isn't particularly traditional. Since you can get munich and pilsner extract now and flaked maize is common, you are good to go.
For mash schedule - everyone has their preferences, but a "medium" fermentable wort is pretty standard. The critters are what really dry out the style anyway. Depending on the mix of critters you prefer, you might like more simple sugars to sour faster or more complex sugars to get more brett in the mix. That is more preference than requirement. With extract, that choice is made for you and you'll only be able to adjust the mix by varying the timing and amount of the various critters.
Malliard reactions are actually desired in many styles. Most of the commercial brewers are doing 2 hour boils to get that subtle kettle flavor.
Many sours are purposely unstable. I mean, you are dosing them with bacteria, right? They also are aged in barrels and exposed to o2. You don't want a ton of protein in the mix, but most malts and extracts are low protein already.
Honestly, the bacteria and "wild" yeast that make traditional sours are some of the toughest organisms on earth. We spend a ton of time cleaning and sanitizing just to keep them out of most of our beer. You really don't have to worry about them thriving in the wort. The only real "trick" is getting the balance of those various flavors to your liking. I've tasted some extract sours that did a great job of that (to me at least) and some AG sours that were just bad. There are no shortage of ways to make good, or bad, beer.