I'd definitely pursue the skylight idea, it will make a huge difference to the feel of the room. Particularly a skylight over a bath/shower can make the room look really great and is easier to install where the height to the pitched roof is shorter. The window is very narrow, not sure as it's kind of quirky & charming, otherwise replace with another wider old-style sash window or remove all together and just use the large skylight and good downlights. 1800mm wide is lovely for a bathtub against the back wall, and will make a nice focal point as you walk through the door and change the bathroom from looking like a narrow corridor as it currently tends to. Back wiht some fresh white subway tiles which will feel lovely in your farmhouse and with that lovely traditional door and some simple traditional tap ware/showerhead. I like your built in bath reference, I know feature stand alone baths are popular, but with small children, I'd forever be mopping up awkward spaces around it. You maybe able to reuse your existing bath if it's in good nick. Then a lovely vanity with recessed mirror cabinet built into the wall for lots of storage. I wouldn't fuss too much wiht the toilet, I'd keep it as is, tucked in the corner.

something to keep in mind is the thickness of the walls and correct installation of the in wall cisterns and best practice for plumbing and noise control in adjoining rooms so consult your designer and plumber to confirm suitability of any arrangements but for our two in wall cisterns we have a thicker reinforced stud wall in one and the other has the cistern built into an extra cavity in front of the stud wall with a section of counter over the top ....my suggestion would be to have the powder rooms arranged as follows with the option of the extended counter over the extra cavity for the cistern if necessary

You have posted on the Australian houzz site. But we can still help. Which would be used more, the desk or the guest bed. If the desk than I would have a sofa bed, which can be pulled out when needed, but a sofa when not needed. Gives you more room for a home office. The photo you have posted of the bed, that is a sofa bed. If the room would be used as a guest room more often, then make the wardrobe space into your study area. When guests are there, you can just close the doors.

I like the 2 colours individually , but personally I would paint the ceiling white in an office -- the grape adds excitement , the white is calming , whereas the mustard could distract and almost 'anger' the brain . Having said that , I assume it has cultural roots , and you may consider them your happy places . My suggestion would be to do some cardboard mock-ups -- I use a large A3 colour Laser printer , which is big , but it has 5 papertrays so is only topped up every few weeks . But I like it handy to my 2 or 3 tower computers ( under the benchtop ) and screens and keyboards and phones and laptops , so a design that works for you mightn't suit me , and vice versa .