My Gleditsia is 30 years old and has also remained small until this year, when it has made more growth than usual. I put it down to the extra rain. However, I can still prune mine into shape.
My eucalyptus is a major problem. It is so huge. I really don't know what to do.
Try Amelanchier. It is the best small tree (in my opinion). Beautiful leaves, which change colour throughout the year, covered in flowers in spring and red berries in June (it is called June Berry). It is a twiggy tree which grows very slowly but you can buy one of a reasonable size to start with.The Blackbirds, in particular, love it. This year they visited the tree in June in great expectation and showed visable signs of disappointment and anger because this year, due to the weather, for the first time in 30 years, the berries did not ripen until July!
Cotoneaster Waterii is another good one - still covered in red berries and is evergreen.