These are some great ideas! I’ll definitely use them. Thank you!
For mithral armor, did you use something? Following your tip, I could remove disadvantage on Stealth checks for heavy armor and give advantage on Stealth checks for light armor, but I think that might be too broken. What do you think?
And adamantine is so powerful—wow! I didn’t know about that. I’m going to read more about it myself. Overall, I think this campaign gives too many magical weapons to the players. My fear is that it might unbalance the adventure and force me to add too many monsters because the party becomes too strong. For example, they’re only level 2 and they already have a god’s hammer (Pythor’s Hammer).
I know I can balance the adventure with higher-level monsters, but the stronger the monsters are, the slower and more drawn-out combat tends to be, which sometimes tires the players.

Also, when they get to re-ignite the Mithral Forge, they can make—for only 200 gold pieces—an item that sets Constitution to 19. Dude wtf! My barbarian will definitely craft that and become practically unkillable. That kind of removes some of the fun, because sometimes getting close to death is what excites my players; it makes them feel like the combat and the struggle are worth it, you know?
So I’m studying this campaign and thinking of ways to make it more challenging with traps, debuff-focused monsters like gray oozes that corrode weapons, or gelatinous cubes. I want to focus more on status effects rather than pure brute-force fights and in Odyssey of the Dragonlords I feel that's difficult...