If this were to be my house the first thing I would do is paint the fireplace the color of the wall. It looks rather dated and also makes the room look smaller. If you were to put the entertainment center in the corner it will compete with the fireplace and chop up the area even more if the fireplace stays as it is. One of your first priorities should probably be to decide on a color scheme so that you'll know what to look for in terms of drapes and a carpet. If you find a carpet you love you can pull out different colors from it for your furniture and drapes, which might simplify the process. Having a lot of dark furniture with dark floors might be too much unless you're only talking about wood furniture and not couches and chairs. Just remember you'll spend a lot of time in this room so take your time to pick items you really love in colors that make you feel happy.

I would suggest a much larger and more colorful rug for the sitting area. Your TV setup looks awkward and unattractive. Can it be changed somehow? I would paint the brick wall in the dining room the same color as the other walls to open up the whole space. I would remove that large mirror and put a smaller painting there and also remove the tall bookcase(?) near the couch which adds nothing to the decor. The mantle should be cleared and some much larger items placed there. I don't know how you feel about painting the fireplace the same color as the wall, but that would also make the room look more cohesive and spacious. I'm afraid the ceiling lamp by the front door looks too small and droopy to have any impact. You need something much larger that has a greater presence. Another thing to consider is white blinds instead of brown ones on each side of the fireplace. Again this will open up the room. I would remove that small table to the right of the TV with the little lamp and put a great-looking floor lamp there. Is there some way you can do without the bulky ottoman? I think that's one reason that area looks so crowded. I would think about replacing it with a small but sturdy glass and metal coffee table through which you can see the rug, giving the illusion that there is almost nothing there. Hope I haven't overloaded you with too many ideas. It's your home and you can do whatever seems right to you. If you follow even a few suggestions you might find that some of your concerns are removed.

When was the house itself built? The interior design aspects are on point with the mid 1980s but the architecture is earlier mid century. I can't really picture someone building a house in that style and decorating aspects of it in late Modernism/post Modernism at the same time. I can picture someone updating a 1960-odd house this way.

Congrats on the new home! My husband and I also purchased our first house recently (been in it about six months) and we're in a similar boat (though with our ratty college furniture instead of hand-me-downs), embarking on an adventure to cohesively decorate the place. While I do have good sense of how to decorate and am not worried about going about it, it would have been really difficult even for me to decorate immediately, so I'm not concerning myself with it (we also have a bunch of house projects in the way). Basically, a good strategy is to take your sweet time and not worry about stuff not matching/ suiting your tastes or the walls not being quite the right color right now. Hone your vision and slowly collect pieces that adhere to it, adding to your spacing or swapping something out. Also, simply live in your space for a while. Be by yourselves, have a friend over, have lots of friends over, host a fancy dinner party and also a casual barbecue. Observe how you and your friends interact with and move around in the space - this will handily inform what kind of seating you might need and how you want to arrange things. I'm always taking mental notes when people are over lol.