I think T: Part definitely works. I'm curious if T: Circle is ok though, as Colombae are much better with Touch/Ring/Circle spells than other types of hermetic spells.
My logic behind the Rego was because without it, the spell turns all the earth inside the circle into stone. I only want to create a ring of stone, and so Rego seemed one way of getting the precision. I wonder if adding Magnitudes for Finesse could also work / would be a better way of achieving that.
I don't think Rego does anything added in to this spell. Rego lets you move/manipulate/control the target, not limit how much the spell affects (T:Part, right?) However, you could probably use a Rego requisite instead to take a ball of clay/etc, transform all of it into stone, and then expand/unfold it into a full circle as part of the spell. Rather than limiting the amount being changed, you're arranging and perfecting a circle with what you transform.
I'm not sure about the Ring duration when something is the ring. There are two end conditions for Ring, and breaking the ring is only one of them. Is it inside the ring? If it isn't, does that mean it has left the ring?
From what I recall, a ring is considered to be on the inside of itself when confronted from the outside, and outside itself when confronted from the inside. (At least from a warding perspective.) Thus, it's possible for a ring to be both inside and outside of its own effect, depending on which direct you're approaching it from.
That might be troubling from this perspective. You're outside the ring when you make the transmuted earth in it. You then step into the ring, and the transmuted earth is outside the ring. Thus the transmuted earth has gone from inside the ring to outside the ring, ending the spell even though you haven't broken the circle and it hasn't gone anywhere.
I wonder if it's not that there are three regions: inside, on, and outside. For example, if you make a ReCo ward against people, people inside the ring are stuck inside and cannot affect the ring itself, nor can they get outside of it. Meanwhile those outside of it cannot affect the ring itself nor can they get inside of it. So there is a thin region neither can get to, what I've suggested might be called "on" the ring. Then for this spell the transmuted earth would never have been inside the ring, but on the ring.