So the single-jointed full cheek snaffle bit I have been riding my new girl in is a loaner and the loan has just been called in, unfortunately

... so I am wondering what I should replace it with given that she is a very willing (to go flatout in a straight line) 7 yo thoroughbred mare who has never had a rider on her back who wants her to slow it right down and listen.
I want a mild bit (maybe frenchlink snaffle) and cheek piece (maybe D ring). Any thoughts? And is a bit keeper still useful with a D ring as well as the full cheek?
Just wondering what the mildest, most effective combo would be.

Her new job will be trail/beach horse, with company in the paddock but riding out alone.
Great thread btw.