Hey Norm, some of the article you shared I liked. Other parts not so much. Definitely try to get a boat that fits you and is physically comfortable for you to paddle. Having your own boat means you can be more independent and paddle on your own terms.
There are many factors that go into boat design. The article you shared seems to really focus on length. That’s just one factor in how a given boat behaves. Width, rocker, and hull shape are also just as important. In very general terms the article is useful. You are asking the right questions about how specific boats are designed and what would serve you well. I’m sorry I’m not more help there (very limited rec kayak experience).
The only river I’ve paddled in Nebraska is the Niobrara. The water was up and some folks in rental canoes were struggling a bit. I paddled tandem in a badly hogged out poly rental canoe and did just fine. I would have also been fine in one of my 6 ft ww kayaks or one of my 12’+ kayaks. So it is not really about length. It is more about you. Some boats would require more effort (tiring).
Beginners often focus on finding the perfect boat to get them down the river but just as important is your skill development. Obviously you’re interested in advancing, improving your comfort, and efficiency. Just realize the boat and paddle are only part of that equation. Right now your focus is on the boat/paddle since you are looking to get your own kit. I’m not much help there, listen to the others here. If you get bitten by the paddling bug, your first boat will turn into one of many. Most of us are still honing in on what we like.
Comfort is now my top priority in any boat. I’m personally okay with struggling a bit with tracking or stability when I’m learning a new boat. That leaves room for advancement. I grow into my boats. Sometimes the beginning is a little rough but soon the boat becomes a predictable old friend.
Seat time (practice) is important. There are a lot of good youtube videos on “how to” paddle. Check some out and focus on just one or two things every time you go out and practice.
Practicing is really just messing around and having fun in a controlled environment. It is okay to get wet and self rescue. You just want to do it in a safe environment. Tipping over isn’t necessarily bad. It can be part of a learning curve.
It sounds like you’ve already tried out a couple of different boats supplied from your buddies. Different boats can pose different challenges.
You know you don’t like the swifty. My advice is try some other boats and see what you do like. Keep paddlin’ and keep askin’ questions.