Hmmm tricky, perhaps just Roman blinds hung at same height on the outside. With the panelled hearth you may have to add a little bit of panelling in the fire surround

What is the name of the quartz countertop, will help determine backsplash

Could you get a longer less high firebox?

I don't know what this does to the outside elevation, but I'd consider installing a smaller window.

Or just move the whole firebox and hearth.

mostly the windows are not the same type and not similar in size at all. so the second issue of the fireplace not centered is just more cumulative "things". I would make a vignette around the wall near the fireplace...build bookcase on the right side, as you have space . It will have the appearance of intentionality. If this type of construction won't happen and the fireplace can't be moved, you resort to distraction.....botanical or patterned wallpaper....fabric valances/panels...things hung on the wall/etc. Create things to look at other than the things that don't line up. Just depends on what you can fix at this stage and how bothered you are......people have been disguising irregularities in walls/structures/ for centuries. Ever set foot in 300 yr old pubs....? Your wall will not look like what Irene is showing with posted pics..but be creative and it will be yours. I would be mildly irritated but get on with it.

Could you do a more modern surround without a mantle and do open wood storage on the right?

Groveraxle can do a better photo shop but just to give you a bit of a visual.

I think your carpenter built base would have to go; I’m imagining something like this:

Some white subway tiles but done in unique pattern. Here are a few.

Perhaps do a white built in under small window to balance other side. But I couldn’t live with that offset fireplace. Big problem. Will be a big negative come resale time too so better deal with it now. Big misstep.

pinpoint quartz

We have remodeled to make a living room more pleasing proportion wise and this would make me nuts. I would have the window on the right (the large one) replaced with the same window you have on the left side. That opens a wealth of possibilities for either built ins or cabinets etc.

I know you just built that base but it just adds to the asymmetrical look. Everything is unbalanced. I would remove it and have a thinner base with less height and have it centered under the firebox also bring in your quartz to the fireplace if you are planning on framing it. For your smaller window add trim to connect with the window and paint inside white.

Is the wood floor under that hearth? If not, that creates big flooring issue. So choices need to be carefully thought thru. I would figure out fireplace before doing backsplash. The fireplace is hardest and some black around firebox in granite or stone, plus a cap of dark stone on top of hearth will help ground the space. Then decide on backsplash.

You can also do this paneling to throw off the unbalanced view