I see you have already wired that upper branch down. Good choice as a branch if you were to retain the upper section but, if it is to become the replacement trunk it will need to go back upward.
Here's what I would try to aim for in the next 3-5 years.

Please not you do not have to chop the top now. It may even be better to wire that branch upward this summer and try to develop it into a mini apex while the top is still on the tree. Reason is that when it becomes the top it will tend to grow stronger, with long internodes. While the top is still dominant that branch should grow slower with shorter internodes and will be much more in scale when you replace the top. Also less chance of dieback if the replacement leader is a bit stronger.
Downside is the chop. Cut now and it can start to heal over while the new top grows. Later chop will mean healing will be correspondingly further down the track.
I wonder whether that Ebihara partial cut method might be worth trying in this case?
Please note that my experience is that JM do not look great if chops are made straight across. It may initially seem a good idea to leave a side branch but that seems to look awkward in the longer term, often developing a swollen area.
I get much better results with a sloping cut through the node. Leave the branch you need as replacement but remove the opposite branch. That may seem to be counterproductive when you really want a side (or back) branch at the bend. Rest assured that more than 9 out of 10 new buds will emerge from the branch collar and one of those can be developed as the required side branch.
Here's the cuts I would make - seen from the side

Note the chop on the rear upright branch - for the same reason as the minor trunk chop - to develop bends and taper.
Both rear and 2nd trunk could be chopped this year to begin developing replacement trunks.
You may find that Spring shoots are strong, with very long internodes. No use for what you need. Usual advice for trimming JM is to cut shoots back to 1 node but that's no use if the first internode is 3 or 4 cm long. Do not hesitate to remove shoots with long internodes completely. New shoots will soon emerge from the basal branch collar. Initial growth in Spring tends to be long and strong. Replacement shoots that grow later in Spring or early Summer tend to be shorter with close nodes - much better to develop ramification on smaller bonsai.