Hi, trunk chopping is generally recommended to be done above the lowest branch or branches as getting new ones below that can be a problem (as you found out). Not all trees will 'sucker' (grow new shoots from roots), but some deciduous ones do, particularly apple trees. What you can do is just continue to cut off the root shoots, but if you get no new growth from the trunk, you may want to consider leaving 1-2 root shoots to develop, along with any roots they may grow on the underside of the original root, and eventually cut that root off and grow it as a new tree. Whether or not the shoot becomes the 'tree' you want, I can't say.

Cheers!

It's risky affair trunk-chopping if you can't know for sure whether it will give shoots on the trunk or not... By the way i've let one 'sucker' to survive to develop into a tree, i had no choice.