I agree that a 6-foot tank can sustain an emperor angel for many years as it is not a fast growing angel. However, I do think that once it reaches 10", it will need to be placed in an 8-foot tank to be happy.
I have also seen a 12" emperor angel in captivity that was grown from 4", so it's possible for them to reach over 10". It's just not commonly seen because most hobbyists keep them in tanks that are too small so they never reach "show size" and eventually die from stress of being in small confinement.
I don't think a 6-foot 210g can sustain an emperor angel for its entire life span of 15-20 years, but if the OP will take responsibility and upgrades to an 8-foot tank when the emperor reaches 8-10", then I don't see the harm of keeping it in a 210 for the next 4-5 years.
As a reference point, I have a 11" French angel that I grew from 4" in a 225g. By the time it was 8-9", the tank was obviously too small and the angel's behavior started being awkward. It would hover around one area, and then quickly dash to the other side of the tank. That's when I realized it was not happy being in the small tank, and upgraded to an 8-foot 375g. Quickly, the angel started behaving normally and grew two more inches in 10 months. It is still growing I believe.