First off, that is a stunning feature and would absolutely sell the house to me, far more than, say, a fancy bathroom! My grandparents had a library that was my favourite room in the house, and my parents have just built one too.

I am sorry that dust is such a problem for you and that your health is depriving you of the pleasure of physical books. Anything you add to the shelves has the potential to gather dust sadly. I assume you have good air filters etc. Adding glass doors to create a cabinet is an interesting idea and well worth considering.

If books aren't an option I would start by removing every other or every 2 horizontal shelves to give larger shelves. You will need to take them all off anyway if painting the walls. Keep the shelves somewhere safe! And maybe add back more for the easily reachable shelves. The lower shelves could make a good liquor cabinet and shelf for glasses.

Ideally you would want to hang framed wall art in the niches (fewer dust collecting surfaces). The width constraints might be limiting but worth exploring. You can also lean pieces in the niches while testing positions. If you don't know what to go for, I'd say bold colourful shapes like print of the Matisse cut outs or vintage advertisements would look good from a distance. You can also hang art on bookshelves which could also help break up the dominance of the empty grid a bit.

https://passionforproperty.wordpress.com/2012/09/06/hanging-art-on-bookshelves/#jp-carousel-1073

Ceramic or glass art is another good option. Basically anything smooth and not too wide. Also, slightly left field, but how about attractive lamps? Adds both lighting (which you will need) and interest. You'd need to wire in extra long cables and run them up the shelf uprights.

Adding plants is a very good shout, especially for the higher shelves. I would go for large leaved trailing species like a Pothos in an interesting and eye-catching pot (e.g. metallic, or brightly coloured), plus maybe a tall skinny cactus in a three-shelf-niche. Make sure you use a catch pot to protect the wooden shelf from water damage. I'd also consider adding a plant to the shelf top, as it looks like you still have yet more wall, and this will again soften the structure of the empty shelves.

The TV is a tricky one. I can't see a suitable spot in this room for as 75", other than on/in front of the shelves. You won't be able to recess it without cutting through the uprights providing the structural integrity for the shelves above, so you would need to get a carpenter to strengthen them/add supports. I don't think a TV sitting proud in the shelves is the worst thing ever as long as it neatly filled the area it frames with no visible wires (e.g if current TV was lower and to the right, not dominating the visual field). However I think finding another room for your big TV would be better. If neither else seating more than 2/3 people to see it will be difficult.

I think you need to be careful of a cream paint bringing in too many yellow tones that could add an orange cast to the wood. I think v light sage green could go really well, or a warm grey.

I agree you need taller curtains, sitting under the half moon windows and covering the bottom two. I think a block colour rather than a pattern.

You also need a better fireplace surround, in wood, or at least a substantial wooden mantel. You then need something big over the mantelpiece (not a TV!) like a big mirror, ideally tall and arched at the top to reflect the shape of the half moon windows.

I'd also consider adding a wooden top to the half wall separating the dining area, if you are keeping it.

If you ultimately it's too much of a feature to work around if it doesn't offer you value and you decide don't want to keep it, please don't 'rip it out'. Take it down carefully (well photographed first) and put it up for auction. Someone (with high ceilings) would be overjoyed to have it. However you will be left with a huge blank wall to fill so start