Another thought is to get a middle buster and remove the plow part and take it easy (few inches at a time) and make several passes. Then use the plow part to open up the trench. Since you have a HST, you can take it pretty slow... Good luck with it...
In the last couple of weeks I've dug about 150 ft of ditches for drainage pipe in very rocky Northern NY land.
I purchased an Everything attachments subsoiler and a middle buster point (not the entire attachment). The bolt holes for the tooth on the subsoiler and the middle buster are the same, although the middle buster doesn't have a flat section on the blade where the bolts go through, so you really have to tighten it down well.
Regardless, I've had very good luck making multiple passes with the subsoiler first, to loosen up the ground, then change out the subsoiler shank for the middle buster, and come back and clean out the ditch.