Yamazuki

Which then prevents us from using melee with fire input even when turning on melee mode, as it now very slowly fires one shot every couple of seconds rather than using the melee weapon.

Many grinds in the game are not tied to killing enemies in high quantities, not using an 'aoe gun' in many situations has absolutely zero impact on the time it takes to earn the reward for the given content cycle unless you're going to convince me that wasting time stock piling resources that have literally zero use is somehow an efficient use of time. It doesn't matter how optional killing enemies are to the grind, people who use 'aoe guns' are just going to use them.