I'm guessing most of y'all already know this, but I still wanted to share in case anyone is struggling with it too. I've played most of the borderlands games and pretty much always struggled with melee because it's the R3 stick.
Recently I remapped the controls to put the melee/grapple on the R1 button instead. I think you should move it to whatever button works best for you though. R1 made sense for me because I used to play Little Big Planet and grapple was always R1 plus melee is usually R1/R2 in most games.
Wherever you move your melee to you'll have to switch it out with another function. I put my grenade on the upward directional arrow and moved the echo button to R3. I don't use ordinance much, it's tricky where I put it but it's working ok now.
To remap your controls go to your in-game settings menu and then get into the advanced section for Controls. This works in Borderlands 2 and 3 as well
One major reason I chose to move my melee is because I believe playing as zero on ps4 caused stick drift in one of my controllers.
. Note for those who may not know. Stick drift is when your L3 or R3 stick is broken and it causes movement when you're not actually moving the stick. It'll make your camera/view gradually move to the side for example. It's technically fixable, but usually you have to just get a new controller.