You can certainly get the look that you want and depending on the window and performance that you pay for, the performance offset will not be that bad. Most 2x4 framed walls perform at and average R-Value of about R-7 to R-9. There are window systems that are R-6+ without particularly exotic materials. In conjunction with a high performance window, you would be well advised to consider some additional thermal shades or curtains to slow additional heat loss from the space.

Making a tree? That’s awesome. You sound quite creative so here’s a thought. Why don’t you use plain shades, as the others have suggested, and paint your window trim the wall colour, so it all just becomes background. Then you could paint or buy a simple mural (branches, vines, woodland stuff), and flow it right over the sill and trim, on the wall? With a 3D tree, it could be quite spectacular. I’m not exactly sure what your theme is, but you could even hang stuffed monkeys, birds, fairies, etc. (whatever suits your design). Here are a few pictures: Jungle https://www.houzz.com/photos/jungle-room-tropical-bedroom-chicago-phvw-vp~270556 Garden https://www.houzz.com/photos/little-girls-bedroom-traditional-kids-minneapolis-phvw-vp~112598 Monkeys https://www.houzz.com/photos/eclectic-kids-eclectic-kids-raleigh-phvw-vp~1176668

Back before the days of air conditioning, it was common that houses of all kinds were designed to accommodate the regional weather conditions. In the U.S. north east, where it's cold early and often, houses were small, inwardly focused around a central fireplace, low ceilings and few windows. In the U.S. south and southwest, conversely, houses were expanded, the "dog-trot" and "shotgun" regional houses were developed to facilitate cross ventilation, houses had high ceilings, many windows, and as many ways for cross ventilation and heat loss as could be imagined, including windows on all sides of the house. Regionalism was the driving force for houses. Mr. Carrier changed all that in 1902.

Short term consider installing the temporary pleated painter shades on the top windows, at least, to get a sense of how you'd feel about living with those windows covered all the time. The fabric ones do a reasonable job of approximating cellular shades. Most likely you'd need 4 of the 36" wide, which you can order for less than $50. total. https://www.homedepot.com/p/Redi-Shade-Cut-to-Size-White-Cordless-Light-Filtering-Privacy-Temporary-Shades-36-in-W-x-72-1062495/202617422#product-overview You may find most of the time your view is directed out of the lower windows and you'd be content with fixed shades on top. Not "once and done" but an inexpensive test drive to inform your more expensive long term choice. Don't leave them up indefinitely, though, or they will be difficult to remove.