You can get discomfort in the upper back and neck if the stem is too long for you. Try a shorter stem.
I’ve done 1,200 mile self contained trips, and I can say that a good steel frame will be more comfortable for a touring bike. I wouldn’t bother trying to test ride a bike fully loaded, as it takes a while to get comfortable on any bike fully loaded. In other words, they will all feel terrible if you aren’t used to the load. Once you get used to your bike fully loaded you will understand that it is a completely different thing than a bike with only you on it. After being on a fully loaded bike long enough (more than a few days) to get comfortable on it you will find that an unloaded bike feels squirly, and unpredictable.
With that being said you will want a forgiving frame, and one that can take the load. The fork should have a good amount of rake to absorb road vibration, and you will want a 700x35c-45c tire.
My current “touring bike” is a modified high end mtb frame with a cyclocross fork and adapters for the cati bosses to bring them up about an inch for the 700c rim. I have a Salsa Cyclocross handlebar with SunTour BarCon shifters. I may upgrade to STI shifter/brake levers in the future, but for now this has a nice nostalgic feel. I have old Weinmann concave rims built on Shimano Deore XT hubs with 700x38c slck tires. This is a nice ride, and I really like it for hauling gear.
Anyway, bike touring is a blast and i have met some of the nicest people on the road. On one trip I challenged myself to get a free camp site every night and at the end of 25 days on the road I only had to pay for camping once. It is the best way to see the country by far.
Have fun.
I did a 1000 mile west coast ride last year on a giant road/cross bike. That bike did not do too well for touring, but it isnt meant for that. This year i am doing a 3000 mile xc ride and i bought a 2006 cannondale t2000. It has Sti shifters which i am familiar with, however it is true that Barcons are much sturdier, and cheaper to replace. I love this bike so far, but have also heard good things about the Surly long haul trucker. I have heard some not so good things about the trek 520 which i also looked at. The most importatnt thing about a touring bike is its geometry, which makes it comfortable for long days as well as being able to handle a load. Dont judge a touring bike by how it handles unloaded. Touring bikes are not meant to be racing bikes, they are meant to get you somewhere with all your stuff.