Torpedoes are generally fired in bursts, as you said; never fire all at once unless A. You expect to die very soon B. You are point blank and NEED to fire all of them to guarantee the kill. I find that keeping a reserve tube in IJN ships like the Yūgumo, and Shimakaze, may have a direct impact on your Damage output, but boy is it useful for catching that gearing/ Russian dd rushing you a good ol smack in the face, helping to balance the playing field of your very sub-par guns .
Also, firing at a head on target isn't all bad, fire so as that each tube intercepts the gap between another tube, therefore creating a solid un - dodge - able wall. (keep in mind this wont work at longer ranges..) firing at targets running away can actually be beneficial, you can force them to turn in a direction not originally intended by them and force them into a bad position where your teammates can kill them (you wont get kills this way, and once again long ranges pose an issue here)
My biggest problem with torpedoes, as mentioned here, is whether the individual i am facing is a good player or not, a lot of the time i miss BECAUSE they are bad players, i expect them to turn frequently or react to a threat, so i fire accordingly, only to see them sail on in a straight line with 0 thought given. Overall they are very hard to use, but are useful in so many more ways than just damage alone; Simply firing a spread into a smokescreen will easily flush out anyone hiding in there (this is something you should also keep in mind yourself, always pointing your nose in an expected torpedo threat's direction.) or denying enemy the ability to move onto a cap by "torpedo walling" it.
Overall they are very high skill cap weapons; often more dependent on the enemy's skill at dodging and map awareness than the skill of the user, but boy, i'd never give up my 1 shot torpedo salvos on full Health Battleships for anything!
EDIT: and i forgot, try to get to know the hydro range of anything you may be facing, that way you have a jist for what to watch out for (what ships you should never try and torp against). Find a hydro ship, fire 1 tube, Pay attention to them, if they seem to see the torpedoes coming, wait a bit and then once you know the hydro has ran out, then fire off another round.