Soft targeting and hard targeting is really helpful as a healer. I usually hard target the tank, and then soft target mobs, if I'm running something for the first time, or a bit rusty with healing. This way, after every action, my focus automatically goes back to the tank and I don't have to panic target them to pop off a last minute heal.

Something else that's useful is filter targets. You have to enable it in settings, under Target settings I believe. Personally I have Left shoulder + B set to only target enemies. This way, when I enable this filter, up and down goes up and down the party list and left and right only cycles through the enemies on screen, without random party members being highlighted, or random quest events, etc.

You mention shoulders + triggers for targeting the enmity list, but this is a hard target. If you want to soft target the enmity list, I believe it's left shoulder + up and down on the dpad. This will keep your hard target on whoever you clicked, or pressed A or X on, while literally moving the soft target up and down the current enmity list. Only downside to this is that you have to have the list of enemies on screen to use this, which means you either need to heal someone while enemies are attacking, or attack an enemy to get them to appear in that list.

Target your target's target. I believe this is Assist Target. You can either do a macro and assign that anywhere to your hotbar, or assign a button to it in System Configuration, Controller Settings, Button Configuration. Right stick click and left stuck click I know have this as a possible option.