Re: seater marks,
"it leaves a ring around the ogive of the bullet."
Two things can cause that.
One, if the ring is sharp and shallow the mouth of the seater stem is too abrupt (sharp corner) and that can be rounded and smoothed as descriped above.
Two, if the bullet depression ring is curved and sorta deep as shown in Mr. Suasponte's photos, the neck expander is too small so the force needed to seat the bullets deforms them; the cure is a proper diameter expander. There is no benefit at all for neck "tension" to use an expander smaller than 2 thou, at the most, under bullet diameter.