Am I right? Its now a twin 5-in with the old style director? This might be a bit too 'retro'; but yes, having read the other posts, I'm agreed with Golly that a decent gun is imperative on this ship, at least for the mission its intended for. The ANZAC class was originally planned with the 76mm, but the RAN insisted on the 127mm/5-in, and I would think they had their reasons. looking at the time period, the 'enemy' would be a certain populous nation to the north, and that calls for some punch against other warships plus attack against land targets; this is what the RAN doctrine would include in any general purpose design. Rockets and mortars would not deal with other warships, so I would avoid that route.
As to what gun; of course its up to LEUT_east, and I did originally advocate a 5-in, but yes, if time allows the later lightweight would be preferable; the 76mm Oto is still o.k., as is the French 100mm, but this would mean a radical departure from existing procurement, and it would need to go on a lot more hulls, which is where it conflicts with the existing 5-in. Also, this is not an OPV, so no way a lightweight 40mm, apart from a secondary role as indicated above.