I think you should put the tree in a growing box (
Article about them) or in the ground, but since you're living in a flat you probably can't put it in the ground. It should thicken the trunk.
I'm with René, you shouldn't cut the tree yet, but if you cut it in late November or early December, you shouldn't have a problem with sap flow. This is for Great Britain and I don't know the exact time for your area.
Do japanese maples bud back well? Don't know, but I'd guess they do. Someboby else knows this for sure. So, if you do, it's your chance now!
Zeb