Would like to see a photo taken from your office shooting back the other direction. Think about centering the TV & base units on that wall and moving the chair over by the chaise portion of the sofa where you can plug in a floor lamp behind it. That will make a nice reading spot. You need table and floor lamps so you can turn off or dim down the overhead lights once you get into the room in the dark. Longer curtains at ceiling height will draw the eye up and give more visual height to the room. There isn't much color; art will be key to adding that. Add some "C" tables so people can set down a drink.

Post pics of your room and a to scale floor plan

We did what you are contemplating but thought we would save money and tax Bill. It was less expensive and we designed HVAC to be easy add later. I put loads of electricals so we had flexibility down the road. That was less cost to do at construction. Certain fire code items were needed as well. Truthfully, that was about 2/3 of the cost. So, we saved but then when we finished off that was cash out of pocket. So it’s a choice. If I had it to do again, I would do it all at initial build. Ours was 750 sq ft heated/cool space. And I wouldn’t have had the mess after moving in. Getting sheetrock up to second floor was messy! Good luck.

I have an older 1 story brick home with a walkup attic converted to living space about 20 years ago. These converted spaces are common in my area, in both 1 and 2 story houses. Pre Covid, when DH was on the road most weeks, I would often sleep in the main floor guest room. Our biggest issue with the upstairs space is HVAC. The previous owner added a trunk line from the furnace to the attic but did not put in an air return. They added a gas fireplace which will heat up the space very nicely so a flip of the switch handles that side of the issue. AC is a major issue. The ceiling was finished so we do not know how well it is insulated but not enough. We did spray foam in the attic spaces under the gables but that isn't enough to stop major heat gain in the summer. Right now, we have a casement window AC unit which is sufficient to bring the temperature down to 78 or 79 F but it isn't as quiet as central air. We knew that HVAC would be a problem. We considered extending supply and return lines but that would have been costly and taken a good portion of the main floor closet space. A minisplit system was a better option but ultimately, we decided to wait and see how much it was needed before throwing several thousand dollars at the issue. Before the upstairs floor and basement ceiling were finished, we put in an empty run of conduit from the install location to the electrical panel for a possible future unit. Perhaps a third floor attic would be different than a second floor space, but I don't find it an issue to run upstairs for this or that. It isn't really any different than going down to the basement for my sewing space. If you have mobility issues or need to answer the door often, the answer might be different. For me, the physical separation of the space is a positive aspect as I can focus better without the main floor distractions like fridge, snacks and tea kettle.