Here's my .02
Ride what you have until you figure out if you actually like riding in the cold and snow. I have known some to say "oh sounds like fun" till they tried it, then they hated it.
More important than bikes is learning how to dress properly.
I have ridden my bike in all sorts of weather and snowy conditions. Fat bike makes it easier. But is not necessary. I would opt for studded tires before fat tires.
Falling in snow is fun. Falling on ice is not.
6M in the winter: even if it is below freezing, if the sun is shining, the open areas will get muddy and VERY slippery. Think slime on top of ice. This not only makes for treacherous riding, but can create ruts that will then freeze and make the trails just awful to ride. Those ruts will have to be fixed by some poor volunteer at a later date. In the woods the trails will get frost heaves that when it warms just a little will melt, making mud. And again, when you ride them, create ruts. Ruts stink.
6M is a touchy place in the winter. Better to find trails that are more rock or sand based, not clay, or stick to the canal path.