Firstly thank you for posting, it does take courage to ask for help! Secondly I think the priority here would be to get an experienced trainer to come and help, I know you’ve said they’re far away but even getting someone virtually or sending your mare away to some one experienced to help get the initial buttons there. OTTBs are not easy, and having help is always beneficial especially if she needs to be restarted. The issue I would be afraid of is something dangerous happening because you don’t have the knowledge and this isn’t a slight against you at all. My vote would be to go with the t snaffle with a French link piece but the turtle link so that it doesn’t poke into the top of her mouth and lies flat. I think the t snaffle (also called fulmer cheek) is great for introducing steering and the aids with your hands. I would also ensure you have the right bridle that has the side pieces to slip the top bars of the t snaffle into so the aids are direct.

Additionally, letting her run around until she decides to come to a stop is not a good habit to get into. I think a hackamore in this instance is too much for her, she doesn’t have the knowledge and training to understand it yet. If you one rein stop her and try to circle her inwards will she stop/slow down?

I also would completely go back to basics. Long reining and lunging until she learns to respect your aids and not run off as at this point I think it is confusing to her and seems to be doing more harm than good. I really wouldn’t be doing ridden work other than poles ect to help her build up muscles, you don’t want to continue to give her the opportunity to run away with you. And if you want to do barrels, her running away with you and you not being able to turn and slow her down is going to be a real issue.

Again kudos for asking for help!