I have very rarely regretted carrying an extra warm layer or waterproof layer that I wasn't sure if I would need - but I have regularly regretted not carrying one. So I would definitely chime in with making sure you have at least a rain shell and some insulating stuff to get you through more bad weather than is forecast. One specific tool I find very handy for planning is the Epic Ride Weather app. You feed it a GPX file and it gives you a time-based weather forecast across that route, so you can have an idea what might be going on at 2am in the mountains.

For food, I personally like to be carrying enough when I leave any resupply point to get me to the resupply point after the next one. That gives me more flexibility in case the weather is questionable, or something is unexpectedly closed, or I'm just not feeling the food available at a particular place. Weight doesn't have much impact on cycling speed, so I'm a lot happier to have 500g of extra food that I carry than to be stuck out hungry somewhere. Same for water.