And I should say our plumber installed the vanity, 4 sets of taps, toilet, shower head and the shaving cabinets. Before the work started the plumber came to disconnect everything which was very quick... Less than an hour.

If you don't want to settle for either colour shade (dark or light) you can always do a mix: frame of your storage light and have the inside of the shelves is dark or the other way round to match your existing furniture. You could even go and make it a great feature wall. Maybe some the images below will give some inspiration. Another option is to "frame" the fire place area by putting some fire safe material behind and under (maybe in the exact height of your black sideboard on the left wall). Storage could be mounted to upper half of the wall. I'd probably try to leave the storage half "open" since the fireplace is kind of freestanding.

Your mockup scheme is similar to our master bathroom we just completed - photo below. I know you mentioned storage being a concern - our mirrored cabinet has offered an amazing amount of storage in a very small space and also helps bounce light around. I was worried about black taps etc too so we kept chrome fixtures but added in a matte black candle and black potted orchid to create a similar effect which we can change out later on. Our floor tiles are dark but because they are semi-polished they actually bounce around a lot of light and don't make the room feel dark if that is a concern. We also have a huge fiddle leaf fig in the corner (not pictured) which adds a really nice feel to the space - a plant like that could help bring in the outside greenery in your bathroom. Instead of the storage tower you have to the left of the vanity could you do say some floating wood shelving, possibly with baskets added for more hidden storage? You could also bring in the 60s theme with what you displayed on there eg. photos as you mentioned? In regards to the window - a previous house we lived in had a window at about shoulder height in the shower - as well as letting in heaps of light it was amazing for storing toiletries etc!

That is gorgeous - nice transformation! Great use of the space you had available to work with :)