An Ohio State blog article recently discussed long-used "volcano mulch" and the problem it creates with the tree's root system...irreversible damage is often the result, unfortunately, in that correction action usually cannot fix any errant root growth. We suggest consulting a certified arborist who has experience in correcting root flare issues (girdling roots, for example), who might be able to determine which roots can be saved (and how) versus which to cut (if any are tangled or curled around the trunk, for example). You probably don't want to remove much actual soil if it's the foundation upon which the main roots are first anchored, but this is where the experience of an arborist would be helpful, since we don't have their level of advanced training. At the very least, you probably just want to keep the trunk's root flare exposed, now that the excess mulch has been pulled back, but it's possible no other corrective action is needed (or possible at this point).

Miri