for the corner Trees for small gardens: Fagus sylvatica Atropurpurea Group, copper beech, is a large tree, but it can be pleached to provide a screen. Tie in young stems to form a framework, then trim annually. Photo by Sarah Cuttle.
Magnolia 'Little Gem'-easily controlled to te height range that suits your requirements ben. Will generally form a clipped block of foliage over time though not as dense as the lilly pilly we chatted about. *Could be used as a "pleached" hedge(st