If the roughest you expect to encounter is smooth packed dirt, then you could do just fine with a standard road bike with 700x25 wheels IF conditions are good and remain that way.

It's when road and weather conditions are not good that going to a 700x35 wheel setup is a great advantage. For instance, going wider on the tires is going to make for a more comfy ride when you start to hit those long stretches of pavement riddled those annoying cross cracks. Pedal, pedal, ouch, pedal, pedal, ouch gets old. Going wider is also safer when you get sand, debris, glass and other junk left on the pavement. Going wider is absolutely better if that packed dirt turns a little muddy or slick after a rain or even when that pavement gets slick with rain. Trust me, it takes surprisingly little to trip up a narrow tire road bike and bring you down. Been there, done that. Then, too, every time I've ventured into a new area with no idea of what I'll encounter for roads, I feel MUCH more confident when I'm riding wider tires. That's why the wider tire bikes are called adventure bikes or touring bikes.

Again, going light is nice and always tempting, but the more miles I've ridden over the years, the more I value comfort and safety on the long rides.

I've been eyeing a Norco Reach steel 105 700x40 bike. Right at that $2000 US price point. It's not available, yet, but it has all the stuff I want on a long distance/gravel bike. Norco is a Canadian company, by the way. I run Norco on most of my fat bikes. Very well thought out designs. Love them.