Brave Axe+ or no?

I definitely would, every point of Mt helps, especially with Brave weapons. That +3 dmg x2 comes out to 6 extra damage every initiation, which is worth the feathers imo.

that 3 attack base variant equates to an extra 6 damage per engagement and that matters a lot more when you want to run him outside of a cavalry team, or in rounds where he is for any reason left without horse buffs.

I do personally recommend it for optimized damage output and completion since i'm a bit ocd about having everything but if you really don't have the feathers...a regular Brave Axe would be fine to fill in until you do have the feathers for it I suppose.

If you want a budget go for the non + but if you want more damage go for +.