A colour called paperbark. It has green undertones so works with both the roof and windows, although will look nice against the garden too. Another is bushland, both are colourbond colours, and available in paint too.

You can borrow wallpaper books from suppliers and compare some you like by holding the samples up to your walls. Be sure to test how well they complement each other at different times of the day under different light conditions. I know it's hard to know for absolute certain how a particular wallpaper pattern will look on a large scale, but it's a place to start. And the people at the decorator stores will be happy to advise you.

Very light grey on all walls would be nice or white. Lighter colours like white tend to make spaces bigger and in ur case this needs to be done. The artwork on the weight bearing wall (in kitchen) needs to go as this draws ur eye to the wall . The railings on the stairs need to be painted (either colour of the walls or white)as this will help blend into the background and make the space look bigger.

I like the colours you’ve suggested. I find neutrals restful. Do you have a sample of your cabinetry? An easy way to get a coordinated look, without whites clashing, is to have the white colour matched at the paint shop to the cabinetry. The colour match will tell you if White Beach is on the right track. I’d consider a more plainer white for the trim ie Natural White as suggested above or Berkshire White. For your splashback - could the budget stretch to the benchtop material? If not a tile colour that picks up one of the benchtop fleck colours will give you a coordinated look. Sounds lovely. Come back and post the completed pics.