Hi there! I have heard of garden designers that do just what you describe. I had a friend who had clients and maintained their garden for them. I'm not sure what her title would be. But she owned her own business doing this. I am sure a google type search in your area could bring up some people for you!
I can't imagine how much time and money I've spent that a person like this could have helped me !! Good luck to you!

Try googling "garden coach" and see if there are any in your area. You might also check to see if there is a master gardener group in your area. They could answer some of your questions and perhaps would know if there are nurseries in your area that offer help with design. We used to have a nursery here that advertised to bring a picture of the area you wanted to landscape and they would help you with the design. Then you could do the work yourself or they would install.

There are a couple of things you can do that will establish what's currently in your garden and what areas need to be developed. First is a shade study. On or about June 21st (longest day of the year), step outside once an hour from 7 a.m. and continuing until 7 p.m. and note where there's full sun, part sun, part shade, full shade. Divide your property into segments and note the light conditions for each. When the time comes to plant perennials, trees, shrubs, etc. having the light conditions lets you do so with confidence based on their individual sun/shade requirements.

Second, draw a map of your property to scale, noting the boundaries and orientation with the compass, drawing in structures (house, garage, shed, pool, etc.) and any trees, shrubs, established garden beds, patios, decks, paths/walkways, etc. Before you start thinking about what to add, it's good to know what you already have. This and an annual shade study were things I did when I first moved into my house. They're also what the garden design books recommend.