Matt4

Your flight is in July so it stills has many time to fill up in J (especially with some business men booking their flight only a few days ago) and you should be aware that the seat map doesn't reflect the load, as many passengers don't bother to select a seat before check in, so your chance to get the second J cabin for both of you only is very small.
If you don't mind (or are delected by) these envious Y passengers boarding the plane and looking at your J seat and your glass of champagne, and don't fear any crying baby in Y front row, just behind row 7 (although you might encounter some crying babies in J too), I would follow that strategy : as you already have a desirable pair a seats and just want to try to enhance your experience with a less busy cabin, wait until 72 hours prior to the flight, when QR IT automatically shows on ExpertFlyer a blocked seat (but still selectable at online check in) for any passenger without a seat already selected. Then you will have a good idea of the load and can think about moving to the second J cabin at online check in (as far as I know, changing your seat selection is not possible in MMB between 72 hours and 48 hours prior to the flight, that's why you have to wait until online check in), but that J cabin can still fill up in the last minute, with delayed/rerouted J passengers, with airport upgrades requested by Y passengers using their Qmiles or Qcredits, or with operational upgrades due to various reasons (oversold Y cabin, delayed/rerouted Y passengers, QR employees on standby granted to fly, ...)
And also, bear in mind that QR can decide at any time that the second J cabin would be used as a Y cabin (J seat, but Y service and Y passengers) if they have more Y passengers to fly than Y seats and if all J passengers can fill in the front J cabin : is that case, all J passengers who selected a seat in the second J cabin would get a random seat in the front J cabin.

So the less risky strategy is probably to keep your already desirable pair of seats ;-)