My first question would be what do you mean by “smart” in this context?
One) do you just want to be able to schedule the lights to go on and off based on other smartthings events ( for example, someone arriving home)
Two) do you want to be able to control the colors of the lights?
Three) do you need to be able to change between display patterns which the tree lights normally handled by a button press pattern?
Four) or something else
The details will matter a lot in this kind of project.
BTW, A friend of mine had an amazing display last year. He set up a tree with tiny dim twinkling white lights and a whole bunch of hanging small mirrors.
Then underneath the tree he had a toy train on a track, and just inside the track running parallel with it, a Hue light strip.
As the colors on the light strip changed, the mirrors reflected the light all around the tree, and the twinkling lights fooled the eye into thinking the mirrors themselves were changing colors.
It was very cool, and not that expensive, relatively speaking.