You better put a DC power supply at the junction box site. You can't do this with AC!
The same thing that makes transformers work with AC makes this unsafe and illegal. There's a fundamental rule:
Currents must be equal in each cable or conduit
It's okay for that current to be split across multiple wires, say you have a double switch powering a light at low/high mode, in both-on you have 1A on hot1, 2A on hot2, and 3A returning on neutral. That is balanced.
It gets a little weird with multi-wire branch circuits (15A on pole 1, 2A on pole 2, 13A on neutral) and gets psychedelic with 3-phase power. But the rule is the same. If you stick a clamp meter around the whole cable, it will read ~0 because currents are canceling each other out.
A "tree" topology enforces this
If you look at almost all physical wiring topology, they are a "tree" topology - one or more cables branch from any point, but no branch loops back onto another branch. This effectively enforces "currents must be equal" because any current going up a branch must come back that same way.
In other words, if you take your drawing and drop it into Paint or Photoshop and do a "paint can fill", no areas should be left un-filled. Note that in your diagram that does not work - the j-box, switch, fan triangle will not fill, ditto light.
If you re-scope your plan with the "Currents must be equal" rule in mind, your choices should be obvious.
Remember you need to provision a neutral at EACH switch location*, even ones which do not yet have smart switches. I presume these switches are not adjacent, if they were adjacent there'd be better ways to do this.
* there are some weird exceptions.