Reduce the damage modifier for balloons, reduce balloon health to scale. (yes this also scales with the helm tools which deal flechette damage)
Balloons break in the same number of shots but can be rebuilt and repaired to full health/functionality faster.
As a solid example, you could set balloon health to 595, and reduce Fletchette and Fire damage multipliers vs Balloon to .75, to maintain a similar ratio of damage-to-health. At this amount, the Barking Dog, Hellhound, and Lumberjack would still be able to destroy a balloon in 5, 2, and 3 unbuffed shots of standard ammo, respectively, balloons will have 75% health upon rebuild, and balloons could be rebuilt in (I think) 6 spanner hits, or ~4.2s instead of ~7s. Other multipliers vs Balloon would need to be reduced by ~50% as well to maintain balance.