Bartre +def - spd, what should do?

If you're using him with something like Steady Breath and Aether, then the effective refine can help, but if not, and you're not running a healer of some sort, then, in my opinion, the refine isn't really worth it.

At that point, I'd rather just run a defense-refined Hammer. That, at least, doesn't cost Divine Dew.

I think the Fury effect is interesting. Admittedly the Res he gains is kinda useless, but he doesn't mind more speed, and really likes more attack and defense. Seeing as yours is -spd, I think double Fury, Brash Assault, and Desperation could be an effective, if not at least fun build. He could bait out an armor, take a bunch of chip damage, and proceed to sweep the rest of the team. If he somehow doesn't one shot an armor, they have almost no chance of surviving two hits, especially if he's carrying Panic Ploy.

On the other hand, you could build him defensively and beef up his defense to absurd levels, like you mentioned. Considering the amount of axes that have high physical bulk (Hector, like you said) I find this is less appealing, but it's still a viable option. Also he's probably better off with another weapon if you're going that route so he could tank not just armors, but physical units in general.

Stat boosts are not worth the divine dew imo.
U can give him a vantage set.
With fury/DC
And brazen atk/res or atk/def bond

He can hit 44-49 def and high 50s to mid 60s in atk. Have him bait out blue/green enemies while he charges up ignis, then blow up on a blue unit.