Building a dedicated listening room

The biggest issue to tackle with any room that is dedicated to audio listening is controlling reverb and minimizing delayed reverb. And by that, I mean if you completely eliminate all reverb in a room it will sound dead and lifeless. As an architect I design all kinds of rooms and spaces that have specific STC requirements and none of them use carpets/area rugs as an element to control sound. Ceiling treatment is one of the most important surfaces to consider. Smaller performance spaces have very different requirements than larger concert halls. Even local music clubs that have great live performances, if you analyzed the space, would be non-starters for 90% of the crowd here I imagine because they weren't "designed" to be perfect.

There are ways to treat the ceiling (gypsum board) without resorting to acoustical panels (which are often applied after to correct acoustics). You could benefit from using an acoustical gypsum plaster finish system. Upfront it will cost more than traditional gypsum but it will dramatically help with acoustics. We use this for large areas/performance spaces and other spaces that require acoustical control without "deadening" the space.  Walls can also be built to have a good STC rating for sound transmission (out and in) but you will still want to acoustically treat with properly positioned panels vs an acoustical finish on the walls.

Soft furnishings (area rugs, comfortable seating, drapery) can all have nominal acoustical benefits but the most important aspect of them is making the space comfortable to be in. You want to be comfortable listening to the music vs adopting your body to a single spot in a room.  You should talk to an acoustician about your space and how you want to use it. They should be able to design and specify the material finishes and help with panel placement (where needed). No space can be perfect.

My forever listening room will also be my library/den which means it will be a rectangular room with/ plenty of natural light for daytime activities and a view towards the pond and forest beyond. My listening position will constantly change in that space but I will have a dedicated spot for optimum speaker performance but also won't sweat it too much as the room is for overall enjoyment of what I like to do while listening to music - read, draw, write and purposefully listening as well.

@8th-note Your room sounds like a well-lived-in, well-loved audio space which is a big, non-quantifiable, aspect of its musicality. I like a room to look as if people live in it vs. a space that is antiseptically designed for one task that must be enjoyed in a certain aesthetic way.

My current room is the open-plan living room/kitchen area of our current home and it's not what anyone would consider audiophile-worthy. That's ok. Most of the time I'm playing music over the speakers as background music. There is furniture and all sorts of family stuff about. But it sounds better than my last space which was more isolated from the house. My wife likes listening to music as background music and can't wait for me to have my own space for my books and music. Primarily so she doesn't have to look at it. So I'll wire the house with wi-fi speakers for streaming background/wallpaper.  And my space will be where I'll spend some time on dialing things in but, like you, will take a laid-back approach to "treatment" because ultimately I want to spend my time listening and enjoying music vs. spending my gray years tinkering with the room.