I see 2 main problems in artillery balance: 1. Colonial 150mm has only one small advantage over warden one, and losing in all other stats dramatically 2. Warden 150mm is just better 120mm, when 150mm techs you basically have no reason to use 120mm
So first colonial 150mm should or have another advantage, for example rate of fire or accuracy, OR increased range on another 100-150m so slow fire rate, bad accuracy and no shields are justified
Second warden 120mm and 150mm should be more different, so in some situations its better to use 120mm over 150mm, and other way around. Something like ability to shoot with 120mm from train carts... can't think of anything else to be honest
Also i think both sides should get emplaced and towed 120mm, asymmetrical of course, colonial emplacement should have longer range, and warden towed arty should have better fire rate