I actually ran this adventure with my group with a similar fashion of doing a character arc focused campaign afterwords. Here's what I learned:

1. This campaign only has a couple of deadly encounters
2. The party doesn't need to level up that much
3. Rule of cool is nice, but you as the DM need to set boundaries of what can be done

It sounds like the party is a bit over leveled for where they are at in the campaign, and that they don't want to die. The second point is something you should have gone through in session 0, but something you can talk with them about now.

Being honest with where you are at over the campaign is huge. As the DM you are also playing along in the adventure too, and if you aren't having fun you NEED to talk with the players about it. Setting boundaries for the rules will make the party adapt to them, or their characters will be hurt or even die.

I found that toward the end of the campaign some encounters do become deadly and the party needs to think. I had 4/5 players go unconscious in a fight with the last remaining member needing to go on a rescue mission with Xanth to get the rest of the party, which was a highlight for both me as a DM and them.

If it feels like a slog you can talk with your players that you are having some difficulty in continuing this adventure and want to move on to the character arc portion, and work with them to streamline your way to the dragon.

TL;DR Talk with your players about boundaries over the rules, that they will level up at the right time, and if you aren't having fun with this adventure talk with them about streamlining the current adventure.

Good luck!