What are the dimensions of the existing room without walls, where are the windows, in the toilet and over the vanity in the shower room? 3M x 3M what are we working with. If you remove all the walls except the linen cupboard and does the linen cupboard also have hall access? If so get the linen opening out of the bathroom and have doors to hall side, which will make both elements work better. I know toilets in main bathrooms are not everyone's cup of tea, but if you can have a 720 door from i take it, a hall, run a custom made vanity unit, and it must be custom made to fit exactly the space available, and take it from hall and over the toilet cistern, which should be a concealed cistern, maintenance panel is the flush plate, and just pan is displayed. Two vanities if needed, one bigger one is more on trend, and hang the cabinets off the floor under the vanity top, to make it look longer and makes cleaning under the cabinet so much easier. That leaves a lot of space for the bath where the second vanity unit is now, which would look fantastic as a freestanding type. A simple frameless glass or semi frameless glass shower screens, allow access to bath, so it might be say 1M wide, if you install those long grid type floor wastes and the shower is long enough you might get away without a shower door. New taps nice big tiles to ceiling and this will look sweet. Close off the toilet and it becomes a bit of a rabbit warren. R

IMO, Adding a full-length mirror next to the bathroom door is an excellent idea and not strange at all. In fact, it's a practical addition that can make the bathroom feel more spacious while also providing a convenient spot for checking outfits or doing hair and makeup. I agree with @dreamer's suggestion that a walk-in wet room is a great option for combining a bathtub and shower in one area. If space is limited, a shower over the bath can also be a functional solution. As the bathroom will be used by three teenage girls, it may be a good idea to consider a bigger vanity with a double basin to make it more convenient and accessible for them. This can help reduce delays in the morning rush to get ready for the day.

based on assumed dimensions my first suggestion would be to rearrange the new stairs (and that will possibly impact the original stairs which you say are already uncomfortable so might also be worth altering) there's more spacious kids' bedrooms with generous biws, desks and separate powder and shower rooms with storage space (towels, vacuum etc or you might like to include an upstairs washer and dryer and a kitchenette in the sitting room for late night snacks or early morning coffee) but i've reduced the size of the master bedroom in favour of an alternate ensuite and more generous wiw and dressing area with access to the retreat without having to go through teh master bedrooms and with more accurate dimensions and correct location of the angled wall it may also be possible to enlarge the retreat and rear balcony by pushing the kids bedrooms to the east and i've left off the extra front balcony in favour of a bigger dual access front balcony but always depends on accurate dimensions and budget

There's nothing worse than entering straight into the side of a vanity. It immediately makes a space feel cluttered/squeezed. A small scaled vanity also makes a room look small (unless it's traditional and you can possibly get away with it). Even if you don't need double basins the extra benchtop is never a negative. Whichever way you do a full depth vanity plus a shower on opposite sides will only give you 600mm between which is too tight. a semi recessed is really your only option unless you have a shorter/single vanity with the shower next to it so you are not squeezed when at the basin. The Japanese soaker bath/shower idea is nice, however you do have to step over the hob which personally not a fan of It also works the best at the end of a room like in the image above. The other option, if you don't want to move the door, assuming window sill is above bench height & are willing to give up the bath, which would feel less cluttered is this: The shower could be comfortably 1500mm In bathrooms, every 100mm and the balance/clearances to/bewteen fittings makes a big difference