AugerTorque has several models made for lower flow units.
I have a friend with a fence company that has a hydraulic post hole digger powered by a 6HP Honda engine. There is plenty of power in the tractor, the problem is flow of the fluid. Most units have been made for higher flow machines, but not all.
PTO augers tend to run 90-130 RPM depending on the gearing. It is an un-weighted engagement and tends to free spin quite a bit so you are not actually digging down at 100RMP.
I have worked with the engnieers at AugerTorque and have a unit coming that should spin at at least 50 RPM. With 50 RPM, downforce and the proper Auger bit/fliting, they are reasonably sure I will outdig most PTO units and with reverse, I will never get it stuck like the PTO models.
Bucket mount will give a lot more height to work with as well as much better positioning control.
For some reason the folks here don't like the hydraulic models. I am not sure why. Most that comment about not liking them don't tend to own them so it is hard to tell why they have a dislike with no experience owning one.
I have run a PTO unit through hundreds of holes and look forward to getting the new hydraulic setup going.