if the fibro is in good condition, simple paint and enjoy your gorgeous vintage home if there's any damage you can replace the old fibro with a modern cement sheet using simple timber battens (looks like the old curved fibro cover strips) and making sure to add suitable insulation...the advantage of using similar thickness cladding is there's no expensive alterations needed around windows and doors although, if you decide to remove the old fibro it's quite expensive to have the professionals do a big job but check the latest regulation and you may be able to replace small sections at a time

Why do you want raised planters? Changing sites is unsustainable practice. The raised walls will be foreign to the site; if concrete or rendered, they are the most unsustainable of all. You will also need to import soil which also has a lot of problems associated with its use. A garden will always look best retaining the original levels. Dont copy bad design, because these bad designs very often break down after a few years. I've been in the industry too long and watched the errors of others, only to see other people copying these flawed efforts.

I wonder how much time you might have to spend cleaning the roof behind the parapet, perhaps the rain will take care of that, leaves?. If you were going to use these spaces as terrace you would need to adjust the height of the parapet or add a railing, if not the height of the parapet could be reduced

I would centre the freestanding bath on the curved wall so both the main features of the room are co-ordinated and aligned. It will make cleaning the floor easier at both ends of the bath. Select your toilet suite to have a concealed cistern built into the wall. Consider swapping vanity and shower locations, with a shorter vanity backing onto the WIR. It is still a small space and you can't have everything large, so you need to work out what is the most important. You still need space to walk around in, to dry yourself and somewhere for towel rails. Best of luck, Dr Retro of Dr Retro House Calls