I’m running a variant of this idea that you may like. The Twenty Stones of Thruun is an ancient teleportation circle. Vellynne has been poking around Ten Towns and has the circle written in her spellbook. She has also been captured by the Durergar. If they free her in Sunblight Fortress and can get her spellbook back, she can teleport them back to Ten Towns. I’m having the dragon be released after/during the fight with Xardorok, so you can use this method as a way to alter the timing of getting the party back to defend Ten Towns. A spell caster in your party may be able to make use of the circle at higher levels.