Reducing recoild helps with aiming, however the game only simulates recoil (horizontal and vertical) and bullet dispersion (which is vastly exaggerated real life values because of game balance) there isn’t much to work with.
There is no system to simulate “it’s harder to aim” as the game assumes all soldiers can hold their weapons perfectly, except maybe by the explosion shock mechanic but adding weapon sway because of a bayonet is a great way to annoy players. Bullet dispersion for using a bayonet is dumb, the bullets will still go where you’re aiming the bayonet does not impact flight characteristics, and increasing recoil is as you say the oposite of what a baoynet does. In my mind, simply cut the feature, people are currently using it as a balance argument against other weapons also getting bayonets and frankly there should be no arguments against letting people bring bayonets for weapons that could accept them.