I'm certainly no expert but I like a lot about your plan. IMHO, it would be good if you could put in a back door on the mudroom. Then it would be more convenient for kids or anyone working in the back yard to come in and use the powder room. The master suite layout is a bit off to me. I would use the narrow area currently designated as the master closet as the laundry room and perhaps you could even block off the end of it as a walk in closet for the guest/children's BR that is next to it. Then you could shift the kids' bath closer to the hallway, and then use the squarish remaining space for your master closet near the master bath. I can see why you might have a library separate from the study. In my mind, one would be a "working" space, i.e. home office, and the other is a gathering "public" space. Is that what you have in mind?

Maybe the door would be good in red? But I always think that. Still.... Or how about a teal or soldier blue door? It's a cute, cute house. Maybe that horizontal rod going across the storm door is removable. I've had storms like that. If you could take the cross piece off, it would let the nice wood door show better. And the light that awmo3 posted would look great on your house. A variegated red-twig dogwood would be good, maybe on the left hand corner. They have nice bright red stems in the winter. (Oh-oh....there I go again with the red.) I'm in zone 4. My dogwoods are the nicest color I can have in the winter. Otherwise it's pretty blah in the middle of winter. You might also consider some Annabelle hydrangeas. They are so pretty in the summer and have a nice old-fashioned look. If you want to, you can leave the big pom-pom flowers on all winter and cut them back in the early spring. The blooms turn rusty brown color and it gives you something out there in the winter. And also one or two small evergreen shrubs would be nice out there so you have some dark green color during the winter. Then plant some daylillies and some asiatic lillies for summer color. If you expand those planting beds out a ways so they are deeper, and make them a curvy shape, that will add to the look too. If you leave the stained porch as it is, you could get some golden or medium brown cedar mulch for the garden and that will tie the porch color into the landscaping.

If you can't change the window to larger, maybe you can balance it with shutters or something... but a look on Google at mid century homes, most seem to have all different sized windows. They didn't matchy match back then; they built more from the use inside out. The door color could change to a contrasting color. I guess people here are liking the exterior siding but I am not a fan of the vertical. But I guess it's in keeping with the style. Also the people who said no to stone. Actually, there are a lot of mid century homes WITH stone; and a lot with brick too. Most of the homes have a mixture of the two. I think it'd be beautiful and you could do a fence of wide wood horizontal planks.... Contrasting color on the roof line and around door and windows... Maybe plant some square boxes of flowers.... you could do that under that bedroom window to balance it if you can't make that larger. Look online for homes to see what they did. Go through the neighborhood to find others and maybe get ideas. Attached a photo of one that is a little like your home. That's how they updated. That wood fence/partition in front is very mid century and in vogue right now.

https://www.homestratosphere.com/blue-house-exterior-ideas/ Sorry the original link didn't work. These are some that similar to my original suggestion.