The problem like always is the meta build. If you run a weapon and just mod it casually. without combo counter, combo duration and attackspeed the melee weapons are fine.. But if you abuse combo counter and attack-speed the stuff scales to hard.
This is a "MMO-Lobby-Shooter" not an "Assasins Creed- Rogue like Knive game." Its weird by design that a melee weapon can be better then a Primary in a Shooter!!!
Just make melee weapon an utility tool, make melee weapons m,ain purpose finisher attacks and extreme high single target damage. A melee weapon should not be able to zoom through multiply enemies. that is the main problem. With your "Gun" Weapon u can only shoot 1 enemy unless u play something special or charge attacks.
Pros of melee over "Gun"/Primary:
dont need aim/super easy to hit
need to be "close" to enemy aka melee
Stahlta is a good example:
i make 1 autohit/slide attack with a melee and 5 enemys are dead. Or i shoot 5 times and aim 5 times to kill the same enemys (assume we oneshot them).
If you dont use the delayed Charge attack, the Stahlta is always not worth using compared to melee attack.
So, how to fix melee?
make all melee weapons only able to hit 1 target (if you dont use charged attack) so you need to click atleast 3 times if u want to kill 3 enemys
make combo counter dmg bonus only work on finisher/stealth attacks.
make an "pseudo ammo" for melee called stamina. if you attack to fast you have to rest a few seconds. (like more then 10 attack per 20 seconds u have to wait 10 seconds until u can use melee again)