gerrr there are some paint ideas to fool around with that I have discovered. I frequently use that recolor tool, can't remember the name of it. There are some paints, or maybe wall papers, that have a different bottom and top. The bottom is usually a wood plank, similar to what you have used in your house. If you drag the wood plank up to the flat area it becomes "siding" for much less cost, and can be colored any shade of wood or paint you want it to be. I think part of what is distracting for me is the wood you used. It looks too flat and too much like flooring, especially since you used the same wood for the floor over the sun room.
A personal "cheat" I use is I browse the different neighborhoods and if I find a wall paper, paint, exterior I like, using that same recolor tool, I save the design I like in my favorites, (the heart icon), or in paint, paper, etc. Then I can incorporate the same design in my own home. Doing this actually has helped me learn how to use that tool better and has given me ideas of my own for recolors/redesigns.
On the floor over the sun room, if you add a bit of fencing around it, it will feel more comfortable and finished. I keep getting a bit of vertigo thinking my Sim would fall right off that. Add the fencing, a couple of lounge chairs, perhaps a table between with an umbrella on the table or a patio lamp, or even add an easel or telescope. That fair folk love seat you used below could also work up there. Add some of the house plants you used IN the sun room, ON the sun room around that love seat. That particular love seat screams for more plants around it.
Something else that hits me is the chimney smack dab in the middle of the front of the roof. You can't see it from the outside of the house itself so it feels a bit awkward and leads to the vertical look of the lines of the house and roof. That vertical look is enhanced by the windows also. Try windows that lay more horizontally, or are a bit more square.
Nothing wrong with using big, wide, windows on the rest of the house as you did with the sun room. Not as many of course. I think the narrowness of the windows you used in the rest of the house is the difficulty.
The tree or trees I see feel lonesome. Can you move them to the corner of the house or cluster three together? They can be different elevations.
By the way, I liked the hydrangeas. Yes, they could have been moved a bit so they weren't quite such a straight line but I liked the color. Sometimes when I'm using them I tend to go overboard with them!
When you are doing trees, or even something like the hydrangeas, try keeping a visual triangle in mind. Like, take a V and put a tree at each point, or a shrub at each point. You can put a bench or sculpture in the open end of that V and fill in gaps with some terrain paint AND smaller shrubs, flowers or grasses. There aren't many grass types besides pampas grass and bull rushes but they and the lavender often work. I'll try to remember to do a few and put up the photos.
In general, your house was pretty cool and has given you a great opportunity to learn all this new stuff. I still struggle to learn and remember tips I get here, (I have post it notes all over my pc!), so in general I'd say you did a great job and obviously are brave enough and willing enough to try all the things the folks here have mentioned. Kudos!