Yeah, the soft targeting is a convenience feature so you can throw one-off heals and buffs at occasional targets, then turn your attention back to whomever you're supposed to be focused on. Confirm will convert to a hard target.

If you're primarily concerned with targeting the tank and the boss, you can bind a target-of-target button. On fights with just one target it should jump back and forth without any problems, since, ostensibly, the tank and boss will always target each other. There's a keybind or you can macro

/ta <tt>