We could bump the Str and Con for the Great Wyrm, but it doesn't fit so well with the rest of the progression. Alternately, we could put the great wyrm CR at 25.
Let's go with the Silver's LA progression.
Eh, ok, it's not a big deal to me either way. As you wish.
Yes, and yes.
Based on the original monster, we should have something like
Tactics: Dune dragons are typically content to avoid contact with other creatures, preferring to hide from contact. However, they are implacable foes when irritated (especially if their treasure hoard is disturbed), using their petrifying breath weapon as often as possible. ....
That'll work.
Description: A dune dragon looks out at the world through azure eyes set in a viper-like, diamond-shaped head. They have long sinuous bodies with a forked tail and short, powerful legs well suited for swimming and burrowing. Dune dragons range in color from white to a sandy brown.
I just noticed while reading the original monster that they have a 90% chance to surprise by popping their heads out from under the sand. Your tactics certainly reflect that, but we should add something mechanically. Maybe a (large) racial Hide bonus when buried in sand?
I'm not sure there's enough justification for tremorsense over and above blindsense, but I'd give them the croc-Hide bonus. Does that work for you?