Our plan was to get the bracken cut before Christmas, leaving enough time for the spray to take hold but removing the fronds in time for frost to damage the roots.
We don't have our own topping equipment yet, and have been let down by our contractor who was supposed to cut it. So, we are now in the process of raking/tearing it into piles to burn, but as ours was about the same height as yours when we started it's a knackering job. I now don't think we'll get it all cleared, but I'm convincing myself it's a scientific experiment and we can compare the cleared/non-cleared areas later in the year.
We are on a reasonable slope which has proved difficult for machinery, but mainly because the bracken is hiding the remains of old stone walls which understandably people are wary of with their machinery, especially when they get tipped over