There are a lot of companies that do flat pack units but will also install for you. I had these units made for our rumpus/media room (flat pack) and the top was our dining room table that was too large for our current dining room but loved the recycled ash. I am sure there would be companies that do this all over Australia.

I agree with above, go with shelving that spans the full width (but not necessarily full height if you don't need that much storage - anything you put up will probably affect the usefulness of the ceiling light you have in the nook). Plain white shelves like this would go fine with the other shelf you have

I think they look beautiful. You'll need to think about placement so they are not located where your grandson might be tempted to climb up to reach them or even try to climb them, but so they don't look wrongly proportioned for the space. I think shelves are a great idea for photos as you can change them around without all the hassle of measuring and redoing wall hooks, although I have to say I also love a well designed photo wall. Are you wanting to combine photos and other display objects on the same shelf or use specifically photo ledges with a groove for the frames? In the walk-in robe I'd guess you might have various possible solutions apart from floating shelves? dreamer is right, proper installation is the key. You need good quality shelves attached to studs. I've seen shelves advertised as suitable for attaching to the sheeting but with the proviso that they have less weight capacity. I'd go for attaching to the studs, which means finding them and hoping they are in a position which suits what you want to do. Which is why floating shelves are still on my 'to do' list.

This isn't a personal recommendation as I have not yet purchased anything from this company, but I've been admiring the timber floating shelves on their website. https://shelvingshop.com.au/product-category/floating-shelves/wood-effect-finish/ Or you can buy the brackets and have the shelf custom made in whatever material and size you want. https://shelvingshop.com.au/product-category/brackets-2/floating-shelving-brackets/  I think hardware stores also sell brackets by themselves. I was thinking of going the custom route because that means I can ensure the brackets will be attached to the studs. I believe you can paint melamine after proper preparation of the surface, but I doubt if you can stain it.