For a starting point - Buffed burst hwacha currently destroys the armor or hull of a spire with 21/24 shots maybe reducing the explosive damage to make this 25 or more shots would be a good starting point for thinking about the balance in numbers.
Another thought - How about reducing the rate of fire:
-Biggest impact on using burst rounds
-Maybe makes greased a real consideration (see my every other post for opinions on how ammo choices/variety should be)
-Allows for more partial evasion
-Allows for armor rebuilds to cancel some shots more often - effectively a nerf to hull damage
-Raises skill level somewhat
-Reduces the proportion of time spend waiting for reload
-Nerfs DPS overall
The downside to nerfing the kill potential on the hwacha would be that there would then be no viable novice galleon or goldfish builds, which are all rather painful to crew already. I would say in that case the spire should replace the goldfish in the novice ship selections, IMO it is easier to use already (200 extra hull health be praised).
Perhaps a small arming time could help, too? Due to the jitter, would it not mean that, if there were a small arming time, pincushioning a ship with the entire clip would be virtually impossible?
There's also the idea of hitting the arcs with a small reduction too. Doing so would increase the chance of evasion even for the more cumbersome ships and increase hydro/chute's usefulness when timed correctly.
However, I do like your idea of a reduced fire rate. Although, it would be sad to see, as, despite the simplicity, I still find a lot of fun in letting loose a completely overkill stream of rockets. The gun feels powerful to use. Nice for the ADHD-addled mind such as my own.
Lastly, your point about a lack of novice builds for ships based around heavy gun placements is pretty viable. Making the hwacha a gun you have to think about would probably just increase the amount of people reliant on the standard metamidion, as it'd be one of the only builds with a simple set of instructions on the box. I know I started welded to my metamidion back when I played in the novice matchmaker. However, there has to be a better way of making a novice-friendly heavy gun without breaking one of them.