I find that magnolia back bud pretty easily and you can pretty much remove 70% of the roots and they'll live (for at least a year, not sure about longer periods, still finding that out). I did my repotting and hacking in the same year, with not terrible results, twice, so I think the results are similar for any magnolia. It's always better to go slow, of course. But as you've hopefully noticed by now, that's not really my thing.
Just as a reassurance: No matter what it looks like now, you'll have plenty of options in the future if you do nothing now.
Looking at your material, there are three equally fat branches, and basically a double apex. You could cut back to the first branch, the second branch, remove one of the leaders. But healing is slow, so it might be better to do such things in spring. Otherwise the wood might rot over winter.
This will give you a basic structure to refine by pruning and wiring.
At least, that's how I'm going at it. I made a solid backbone by cutting large branches strategically, inducing movement in the trunk. And from there on forward I spend most attention on which finer branches need to go where.
The backbudding is unreliable and unpredictable, as is the dieback. So on one of my magnolia, I'll have to start from scratch in a year or two. Or well, actually there are already new shoots forming, so it's more of a continuous process. But unlike most conifers, magnolia seem to give us a couple extra chances. I wouldn't be too afraid to just get in there and start removing stuff.
This winter it'll show you the whole structure it's been building and it's a good moment to grab some chalk, find a front and mark some branches that you'd like to remove.
I'm just undecided whether to go for bonsai with it or not to be honest
lovely little pink streaks on them. 
