If you want my opinion, you need more height in your garden. Perhaps
the photos do not do it full justice. If they dont, then you are
underselling yourself.

I see a shed and some blue fences, you need some vertical lines to
break the monotony of the fence-top horizon.

You can get some good varieties of birch that will not get too large.
They will provide - height, shade, habitat for birds, autumn interest.
Likewise for some (if not most) examples of Sorbus even a Crab Apple
(Malus) would fit in OK. How about a Clematis armandii to soften the
shape of the shed. This variety is evergreen so will be good cover
year round.

These are just some ideas. So why dont you post back with some
feedback on my thoughts.

Oh yes, one last thing - Why decking?