It is never necessary for a horse to be moved out of the snaffle. There are many horses and riders in the western world who are content to go in the snaffle forever. You do lack some of the finesse that you can get in a curb type bit but that isn't always a bad thing.
That being said, so long as she is consistent and soft in the snaffle and is light to your cues, there is absolutely no reason why you can't move her up to the next step. What are your ultimate goals with her? Do you want to eventually end up in the spade bit? I will say that riding successfully in a spade bit is not for someone with limited experience. That has the potential to be an incredibly dangerous bit unless the horse is completely and properly prepared to wear it and the rider knows exactly how to properly handle it. That is why it often takes years to get to the spade. You can always just go ahead and give the bosal a try and see if anything changes. One thing about keeping a horse from becoming lazy to your cues is not allowing them to ignore them. You ask
her for something, if she is a little sluggish then you tell
her, if she still sulls up and isn't reacting as fast as you want, then you demand
that she do it and she must do it now. Sometimes that does mean getting into their face or their sides. It's all about knowing when to be very soft and when to give a good solid pop. I personally will take a horse out of the snaffle as soon as they are soft and responsive and begin to get the idea behind neck reining. However, I take them directly from the snaffle to the bridle.
If she isn't reacting quickly enough on cues that she for certain knows, then you need to remind her that you pick the speed. Sometimes, you can't stay nice to a young horse or they will walk all over you.
The bit that MM posted is a good one but I personally prefer a swivel shanked bit on a green horse, even if they do neck rein (cause sometimes you need a one rein stop and that is very difficult with fixed shanks). If you ultimately don't want the spade but do want something with more finesse than a snaffle, this is the bit that I put all my youngsters in after the snaffle and the one that I ride all my finished horses in.