If there is one tree I do not worry about in winter it's dwarf birch. If they can survive in the high Arctic growing on permafrost soils they can handle the mild frost in my garden.
Summer is a bit more tricky but last summer we had >35C quite a few times and they were happy in full sun in front of a south facing wall. These guys are tough. The only thing is that they like it wet.
The tree in the last photo is now three trees. This is the largest part, starting to form more branches, truck still needs to thicken quite a bit. There is a start is a nebari in the soil.
I air layered a side trunk. That's this now:
One branch I put in some water and it started to grow roots, so I potted it. Might make a tiny tree from it.
So climate is easy, leaves are already small, air layering and cuttings are very easy. It seems to ramify well and grows very fast when heavily fertilised. Downside is slow thickening of the trunk. Obviously it's not suitable for large bonsai as it doesn't get large.