Except we did see the Power Stone destroy a planet. Granted, it wasn't Ronan using it, but I don't see how that makes a difference. Ronan was certainly capable of using the Stone effectively (like when he one-shotted the entire Nova Corps) so there's no reason to think he couldn't have destroyed Xandar if he hadn't been challenged to a dance-off.
True, although the fact that Asgard is flat probably means that its core is shallower than it is on a spherical planet. Also IIRC, didn't Ronan tell Thanos that he was going to burn Xandar to its core or something?
Don't get me wrong, I'm not lowballing Surtur. The guy is ridiculously OP. I just think Ronan was even more powerful.