Looking Ahead

This should be a bridge arc, carrying the story and allowing the characters to gain experience and gold as well as knowledge. The final arc is more intense, and definitely one of movement and action. I see it beginning at the mining outpost of Aron's Bowl, and following the river up into the mountains toward the dragon's lair. Frankly, that is a major element of the final arc. It is as much about the lair effects and the approach as well as the dragon itself.
So, this second act is bookended by the takeover of the fortress and their arrival at Aron's Bowl. Distance-wise, this is a fairly quick trip up the river, 200 miles by boat. The river is easy enough to travel. Moving directly across country, the trip is closer to 150 miles. The route is difficult, but passable. For this section, the emphasis needs to come off the travel, and be about the locations. That means two or three decent side quests.
One possibility is to have them go hard after the cultists. Track down a more powerful hideout and destroy it. The trouble with doing this is that the main quest can still feature cultists as part of the endgame in the final act. This could be a destruction of the minor cult of the white dragon, with the red dragon cultists stronger and still ahead, but it might be better just to skip them so the final run-ins don't start to feel too derivative or familiar.
One good option would be to deal with the slavers on the river. The bandits from early on were kidnapping people to send upriver in chains to be used as forced labor in the mines. The mines can be goblins run, with plenty of prisoners kept in dungeons and forced to work. The party can have an encounter with the bandits, learn the details of what happens, and then seek to destroy the operation by defeating the bandit headquarters, and then the goblin miners, freeing the prisoners. This is a keeper. I can see a lot of fun opportunities here - infiltration, traps, resistance, other (ogres, etc. in the mines).
I think something magical would be a welcome change. Actually, this is the place to use the astral plane hermit. There would have to be something to suggest a hermit, then a quest of sorts to locate his hovel in the cliffs along the way, and finally a way to locate or open an astral gate. Once inside, there are obvious possibilities with Githyanki, etc. Maybe if the hermit is an exile and not just an ascetic. That way, there would be a reluctance to return voluntarily, an active hunt for him once they crossed to the plane, and a reasonable suggestion to delay the quick location of the gate.
I always feel like there is fun to be had fighting giants, and that would be a good third option. A stone giant lair, stumbled across, causing the giants to attack. The lure of glory and gold will draw in the party, and give them a fight. Stone giants shouldn't be too lofty to handle, and what I see is them being based in a series of caves and tunnels.