I really like your draft for its pragmatism. However, I would approach it differently and possibly save some money in the process.
For your requirements, I would think of a barn-like building with a north-sloping shed roof and a touch of loft in the interior. I would make the floor plan a bit smaller and place beds for me and a guest on a gallery, which is very inexpensive with just a simple beam layer as the floor. Soundproofing doesn’t play such a big role in the living requirements. Guest sleeping places don’t need their own room – I like the solution of a "Hems" as it is often built in Denmark. A small carpenter staircase doesn’t cost the world – for the space-saving ash wood stairs to the sleeping galleries in the children’s apartments, we paid about 300€ each. The shed roof facing south is good for photovoltaic use – if not immediately, then later it is a profitable investment. The gained room height is utilized and does not negatively affect construction costs, since the floor area can be smaller. Operating costs (heating) are somewhat higher, but the spatial feeling is significantly better.
From my point of view, a guest toilet is dispensable or bathroom and toilet are generally separated – in other words: one toilet is enough.
Plan storage space very well – whoever knows what they live with and is not a collector can use space very efficiently.
Even in our house, the technical room is quite warm. It is unheated in a room only accessible from the garden under the terrace and even at the current outside temperatures it has about 16 degrees.
It would be important to me to have a generous direct connection from the large living room to the outside.