Once you have furniture in your space the chair rail will not be noticed on that side. It will not be the focal point. Your furnishings and artwork will be what people will see. Also, spaces that are completely balanced are not nearly as interesting as having a little asymmetry here and there imo.

Have an overall plan before you start. Don't get bits and pieces. Let's get some unity, here! Measure twice, buy once! I wouldn't do a sectional - they are harder to decorate around. Better is a couch and a few upholstered chairs. Also, of course, side tables will be needed and a middle coffee table. Wall decor, such as prints or tapestries. A rug will be needed under the entire seating arrangement (unless you live in a hot climate). If you like knick knacks, you'll want to get some of those. Do you need storage (or have room for that)? If so, be sure to take advantage of your vertical space. Secure it to the wall if needed. If you are a neat freak, open shelving is possible. If not, be sure to get closed shelving. I like more space than stuff, but that's me. Will you have a tv in this room? If so, do you plan to put it over the fireplace (which I don't recommend), or on the wall to the left of the FP? If tv is on the wall, you'll need a console under it. You will need to "float" the seating arrangement in the middle of the living room. Allow for enough comfortable space for people to walk around. Traffic flow will be important. Develop an intimate seating arrangement so you can have a good conversation with others.

I don’t know how to take your plan and add furniture to it 🙃... so I will just explain how I would place your sectional. I would float the sectional where the back of one side of sectional is to dining room and the back of other side, is to the kitchen. That way your window and fireplace are the view from sitting on sofa. I think this would be an ideal set up for your space. You could still add a chair in the corner between window and FP, or place an additional chair directly in front of window, without obstructing a lot of your light. It’s a great open floor plan, congratulations on your new home.

Hi @debdecal! What a gorgeous lake house! If it were me, I'd put the fireplace between the patio doors to work with and punch up the symmetry of the windows and ceiling. But it wants to be quite substantial if it's going to be there, even if you use an LCD fireplace. You need to put on the fireplace show. If the masonry/rock/tile chimney effect doesn't go all the way up to the ceiling, it will be dwarfed by the room. If the television must be there, can it be concealed in some way? We don't see the whole flow of the space, but I would probably put one leg of a sectional where your blue sofa is (back facing the camera) and the other leg facing the dining table. (Personally, I would forego the sectional and get a couple of armchairs in that location instead. But that's me swimming upstream.) You can always try out locations for a sectional using the two sofas you have and see where you like them. Best of luck!