I can only answer the first point. Light road bikes are great for going fast with no loads. I don't think it matters if the posture is "aggressive" or not, if it's a posture you like and are able to ride long distances then it's fine. Once you load up on stuff and go steady easy speeds for day to day comfort the light wheels and small tires become less desirable. It doesn't take any kind of fancy bike to ride 6hrs/day at 10-15mph. It's all the motor and making sure there's air in the tires. If you're emotionally attached to the existing bikes I'd go for ultralight traveling instead of trying to pull a trailer or adapting the bikes to carry lots. I'm not familiar with that Fuji but if it's got 23yr old wheels I'd be inclined to replace the rear rim/wheel. Whether the bikes will work for you is hard to say without seeing the bikes. You know, are you 140lbs thinking of carrying 25lbs or are you 200lbs thinking of carrying 30lbs and what condition are the rear wheels.
If you get different bikes it won't be an investment, it'll be an expense. When I had a shop on the coast in the 80's I replaced a lot of rear wheels on bikes people had for ten years but never loaded up with gear. They loaded it up and rear spokes started breaking a week into the trip. So the short answer is "it depends" on the size of the load, you and your gear whether the bikes you have can work.