Hi Courtney and welcome to the Horse Forum :)
Congratulations on your new addition.
The absolute first thing I would do here, is get her teeth attended to, and a once over by a physio. That way you are starting with as blank a canvas as possible and can work on education without having to wonder if pain is a factor.
From there, I very much lik your plan. It is such a breath of fresh air to see someone with a hard mouthed horse, that wants to go back to a mild snaffle, and not put a leveraged, twisted wire contraption in its mouth with its jaw wired shut.
I'd definitely be starting with work in hand. Do you know how to long rein? This may be a suitable starting place for you. Also working at the shoulder. Ask for lateral and vertical flexion, very gradually. Yield the quarters, the shoulders, the neck and the poll.
Under saddle, instead of worrying about directing her with the reins, I suggest trying to establish with her that you are not gong to hit her in the mouth like an unbalanced child or beginner rider. Keep the reins fairly long, just enough to keep a light, elastic contact with the bit. Keep your hands as quite as you possibly can - there is no shame in investing in a monkey strap! - and ride forward. Forward everywhere, let her toss her head, shake it, etc. Just ignore everything in front of her shoulders, and focus on getting her travelling forward. I think at this point you will find that she will start to relax her neck, poll and jaw.
When she is able to travel forward willingly and confidently, on nice wide curving figures, start having a play with asking her to leg yield smaller and larger on a circle for a few steps at a time. Still concentrating on riding through your body and leaving your hands where they are.
Eventually you can start taking your reins and asking her for a little flexion to the inside, a little to the outside etc. To turn, try opening your rein out (left rein if you want to go left, right out if you want to go right) in order to open a gap for her to move into, rather than pulling her there. Work at it piece by picece, don't expect miraculous, overnight fixes, be patient and just continue to chip away.
The point is to break down her defensive barriers. A hard mouthed horse is a horse that is defensively protecting themselves from pain. If you can build the horses confidence by never hitting them in the mouth, leaving the contact quiet and consistent, it is quite amazing to see how quickly their hard mouth becomes a very willing and soft mouth.