I don't do competitive. Keep that in mind as you read.
When I run a brawler Galleon, I almost always run hwacha hwacha. Here are some reasonings:
Your average gunner cannot efficiently utilize Hwacha/H.Carronade. Hwacha/hwacha, any decent gunner can get out a good efficiency out.
Galleon is a big ship with large components. Galleon is also a slow ship. You always want the maximum mobility. You want your two engineers repairing. With Hwacha/H.Carronade, which an average gunner cannot efficiently utilize, an engineer will be tempted (not wrongly) to aid the inefficient gunner. That temptation is extremely dangerous, especially in a brawl. With hwacha/hwacha, your engineer will never end up on a gun; it'll always be reloading.
The most common build in your average match is the metamidion. Galleon is inherently disadvantaged against a metamidion; it's a slow, large target. The best counter against a metamidion is the Hwacha. However, enemy engineers can revue the pyramidion faster than the hwacha can reload. Hence, you want the second hwacha to out disable the enemy engineer's repair.
Hwacha/H.Carro, Hwacha/H.Flak, etc. etc. Are very effective Galleon builds, but on a average Match Made game, Hwacha Hwacha is the easiest, most effective, and most efficient build.