Are There Improvements that Can Be Easily and Quickly Made Without Buying More Stuff?

You have a really beautiful house. Looks like a wonderful view.

I don’t think you need to know what is wrong… or what your objective is. By trial and error you can find out what it’s capable of. As soon I have optimized all wires (which of course is an expense)… then I go after the venue.
 

The answer to your question: all the things you say you cannot change (in your system description). Placement, floor and wall treatments. These can make large differences.

Is moving your system to a dedicated / or at least acoustically better room a possibility? That much glass and open flooring are the first thing I would go after. You can see my system and see what I did was incrementally work on these things.

start w the Jim Smith book mentioned above…it has many…. free…ideas 

I would say go through the virtual systems of other members and get ideas about room setup and placement. The other thing is to look at the recommended dolby placement guidelines for your setup re: the angles and dial those in exactly as you can. The other thing is all of the glass, can you get curtains?

Thx for the comments!  I do have a carpeted space upstairs with only 1 regular window that I could move to.  But then I would have to “do music” as a separate, stand alone activity.  If I did that, I would listen to music much less.

Windows and light are a blessing of our house.  When inside, I’m still connected to nature.  Even living in a city, we daily have deer sometimes coming a nose away from the glass.  And fox and bobcat in the winter.  With a 180deg view of the largest lake in the world 50ft away, curtains would shut out more that I would gain in sound quality.  If I had them, I’d probably rarely use them.  Maybe after sunset in the winter…no, not even then as the moonlight views out over the lake at night can be stunning.

So for me, I’m trying to do the best I can in this context.  Of course, it would be tempting to set up a separate audiophile space with an entirely new system upstairs, and keep what I’ve got here for regular use.  That’s a future prospect.  I’d still want to optimize what I have now.

Speaker placement. Check out the room mode simulator at AM Acoustics. Avoid placing your speakers, subs or listening location from being in any of the lowest room modes. Same for your listening position.

Of course, I am always advocating for good room treatment, and EQ where necessary or desired, but those cost money!