Hi. I don't know what your preferred styles are but I can see a scandi style working if you painted the chairs. Which way does the room face? I might go for white walls ( and shelves/cabinet if allowed). Paint 3 of the chairs in the blue/grey colour and the last white. Perhaps a round mirror over the fire. Align the table with the fire and light. You could add a light grey patterned rug under the table or something with texture. Maybe a chair like this angled in the window with a throw over it. A floor lamp in the left corner by the bookshelves. Perhaps a picture shelf with varying sizes of art ( picking up a little colour) opposite the fire so it limits on ŵall holes. I'd also fill the shelves a bit more to disguise the brackets personally although be careful it doesn't look like clutter. Perhaps more of a statement light? There are some nice star ones around. Also pop something on the table with a little colour. Do you have the budget to add blinds? You may of course not like scandi though!

I assume you like the central groove and so you might search for stacked corbels which seems to bring up lots of availability from America. https://m.ebay.co.uk/itm/SOLID-AMERICAN-OAK-CORBEL-SET-triple-stacked-MANTEL-BRACKET-CORBELS-PAIR/162521706249?hash=item25d70c6f09:m:mzfjPVlQLViCcKZsbBRU9-w I suspect that actually your image shows two shelf brackets that have been put together for extra strength https://m.ebay.co.uk/itm/Wooden-Corbels-Brackets-shelf-brackets/112639077722?hash=item1a39cfed5a:g:BIYAAOSwpHFZadrL

Looks like the desk is too high for you anyway (note cushions on chair), so it’s a genius idea (suggested by Simon Lee above) & by far the simplest idea to trim the desk legs down to reduce the height. It’s too high for comfortable typing anyway & your back will suffer in the long term! No need to move shelves or change location of desk or buy anything else!

Tricky one...is the original worktop wood ?....cant tell from pic. I'm guessing you would have to use a strong angled bracket that runs along the whole length and then a supporting pole. If you just want somewhere to sit for a short while could you sit where the bins are and put bins elsewhere (maybe in a cupboard?)