You have probably been to the tack shop by the time you read my post - lol lol
Others may come in with different ideas, but:
Once you've established that her teeth are in good shape and not getting in the way of a bit, you might try a very low port curb bit (maybe even a copper mouthpiece to create saliva), with longer shanks, and a chain chin strap.
A wise Elder Amishman got me started on long-shanked Walking Horse bits nearly 40 years ago, long before I ever owned a gaited horse. He taught me how to stay exceptionally light on the reins and to stay out of the horse's mouth.
The longer the shanks, the lighter the hands you need because those shanks act as leverage.
My "Rodak Powered" TWH is 20 and still needs those 8" shanks I've worn on him for 17 years. He is a step-pacer, and I retired him, unbeaten, from neighborhood gaiting races when he was 18. While that huge motor of his wears out quicker these days, I still need those 8" shanks to keep his little self slowed down.
I could've taught him not to be so goey, but his personality suits mine to a tee. I like him "up" and rearin' to go, even if it only last the first 20 minutes out these days
I have a 12 yo TWH that only has 6" shanks and also the same very low port the 20 yo wears. He's a lot more laid back than my 20 yo.
I have a 13 yo TWH that has ridden in a mechanical hackamore since the day I bought him 11 years ago. His previous owner said he refused to carry any kind of bit and would perform well with the hackamore. He's my get down and dirty, take-on-the-cliffhangar trails, so speed and control isn't an issue.
If you are planning on doing any kind of speed racking, a bit would be bettter than a bosal, hackamore, or any of the other bitless type bridles.
You just have to remember to have very light hands with those long shanks.
That all being said, it also depends if you want to show and what disciplines you would show in. They may have bit rules. I dunno because showing isn't my thing. I like to watch and try to keep up with the "Who's Who" in the TWH and SSH industries, but that's as far as it goes - lol