It's well within their design to make an oGCD have a cast time. The problem is that because it's currently on the GCD, disabling it is also on the GCD as well (which means you can't acutally "weave" disabling a song) considering the potential uptime and the potential potency of the regens, the cast time for the song itself is fine as it is, just not disabling the song or using it.
Which is part of the problem. We don't want two jobs to play on entirely the same concept, which is almost the case right now. All the two does is keep their debuffs refreshed, keep their own buffs up, be re-active to their procs (which BRD can't do effectively). The only difference I can honestly bring up is the fact that MCH can move better than BRD (which honestly, should not be the only differentiation to the two jobs) and giving straighter shot an increased proc to mirror that would throw it outside the window. They have their own different burst execution, but that only comes up so often in a given fight, that everything else after that have the two executing the same flow of using a filler, watching for procs, using the procs, and using oGCDs when it comes off. There's no management of resources (mana cycling for blm) or limited/building up buffs (like aetherflow or wrath) The ammo system is incredibly straightforward and at best an afterthought since you're encouraged to use quick reload as it comes off cooldown, and ammo itself only has an "additional effect" to two abilities in your entire toolkit, the rest being a flat +20 potency boost. The turret ultimately means very little since it's mostly place and forget, it's not that expansive to how MCH plays as a job compared to BRD (which is different from playing the job effectively).
This is what I need to address. As of right now, do you need barrage to be buffed to the way it was? Before, it was brought up to make a point that BRDs had actually lost damage going into 3.0 because of both changes to 2.0 traits and new abilities that were questionable (WM). You don't need to change old skills back to the way it was as long as it still balances the effectiveness/performance in the long run (which seems to be the case, more or less assuming that everything else works out)
Hitting SS with the proc (but not using the proc) actually delays your oGCD cast, including Empyreal Arrow. Which honestly makes absolutely no sense from a game-design perspective that this is going into bug/glitch territory. Otherwise you're normally supposed to be able to use an oGCD right after the cast finishes.