/equipset 1 (Precast gear)
/ma "Victory March" <stpc>
/equipset 2 (Postcast gear)
This is working for me, I'm not sure what your problem could be, but I'm trying this right now with a macro setup like this, while having quickcast pieces in my precast, and it's working for me when it procs, I'm getting my entire AF3 set bonus from my postcast set.
With the in-game equipment sets, you can't swap from equipment set to equipment set too quickly, otherwise the second one doesn't register, getting the "...A command error occured." message. So, when I was testing this, as long as I hit the macro (swapping me into my precast set) and waited the second before confirming March to cast on myself, successfully allowing the second set to register, I was still getting my postcast bonuses in all instances that quickcast procced. I forced a bit of lag on myself too to see if that may be an issue, was at approximately 15 FPS, and it was still working for me.
As long as you don't get "...A command error occured" from swapping equipment sets too close together, or have any /waits, you should still get your postcast set bonuses.
Quick Magic means your spell casting time will be instant, but it doesn't mean your spell will cast instantly, if that makes sense. Your spell doesn't instantly cast from the moment you activate it or your macro, just when it begins to cast. When you tell the game to cast something, it immediately checks for both casting time reduction pieces reduction and quick magic pieces, before actually beginning. There's still a brief window of time after this immediate check before the start of the actual spell itself, so you can swap equipment in this time, allowing you to reap your postcast set's bonuses.
Beside the obvious equipset error mixups if you have poor timing, this should work the same way as it would with GearSwap or AshitaCast, in the sense that you can precast in Occ. Quickens gear and cast in potency gear. If you wish to test more for yourself, you can use Nightingale(if your's is 5/5) to force instacasts while it's up, and try using this macro the same way you have been and checking for your bonuses from your precast->postcast sets' swap.