Well built steel frames are the tanks of bicycling. Very strong, very heavy. There's nothing wrong with well built aluminum frames. They don't cost significantly more and they are lighter. They are strong enough for all but the most grueling touring. Beyond that, the best things most of us can do to to make the biking package lighter is to carry less gear, and lose some weight by eating better.
You should think about gearing. Road bikes are geared for fast riding. Mountain bikes are geared lower for extreme hills. Many tourers prefer mountain bike-type gearing. It's better for endurance rides (days of 50+ miles/day). It's better for hills. It's better for rides where you are carrying more weight. Think about the riding you do now and what gears you find most useful.
Tire width shouldn't be a big issue. Bikes with connection points for racks typically also include connection points for fenders and also will accept wider tires. I rode the C & O canal before I decided I wasn't all that interested in touring on dirt and gravel roads. I found wider tires are helpful, but more important for riding where the trail can be muddy and rutty is to have some kind of tread to help give a little extra traction. I used a hybrid tire, with a smooth center and treads on the side. If I were to do a lot of road riding but also a significant amount of trails, I would go back to those. But other people in my group rode the C & O trail with narrow street tires.
Finally, you don't have to make an instant decision. You don't have a cross-country trip planned with a deadline start date. You can do some longer rides, see how they work out, develop your knowledge of what you want a little bit. Get off the internet and visit some bike stores. Don't overthink this too much, follow your gut. If you think you like a bike, you probably will. You ride a lot. Use your own experience to guide your decision.