If you bind your slide to right thumbstick press instead of using b/circle, I promise your gameplay, especially shotgunning, will almost instantly improve once you get accustomed to it. The ability to slide (and jump and swap weapons but I don't have any good rebinds for those) without taking your thumb off your aim stick is super important if you're going to keep up with mobile players. As for when to slide, I mostly use it to engage/disengage faster and pretty much any time I want to keep moving in one direction while changing where I'm looking.
You mentioned shotgunning. That's something I had attempted, but as most special weapons, I've put them off my PvP plan because I couldn't get them right.
I have my charged melee bound to that one. That and finisher were the only actions I had remapped.
Because I didn't want to stray too far from the button layout I know from other shooter.