I like. You're making the round reasonable and smaller so presumably launchers can deal with them more quickly.
My two cents on how things are. No drama, just my personal opinion.
Post-FTL speeds, they cannot decelerate or change direction. The forces would crush the torpedo and it's cost in engineering and construction would sky-rocket to survive these forces.
FTL torpedos are glorified self-guiding rockets. They're launched, they aim, activate their engines accelerate to attack-speed and they either hit or they miss and that's the end of it.
If you want to overcome the "it's cheating at close-range!", have them (the torpedo) accelerate gradually, able to achieve high speeds for far-away targets but remain at reasonable speeds over reasonable distances, tracking until right before they hit the really high speeds that would be silly at close range so they remain accurate.
This would bring ships closer to each-other and I imagine fights getting more interesting because it'll be less like sniping - with armor roles really being especially useful for supporting ships - which is great for players.
You dump 4 or 5, they line up and guide, like real torpedos do and when they're lined up reasonably, they switch to FTL, moving STL until then. At close range, they become STL torpedos.
...unless the dedicated launcher gets the round up to speed early and aims for the round. Ships with these "sniper" weapons could augment the huge beam arrays as a tactical class of starship with high-precision torpedo style weapons for striking outposts and enemy ships to disable them.
Space-use torpedos are useless in atmospheres, if not utterly suicidal: Their targetting systems have a wide aperture (lockon prior to launch) and they aren't designed to turn around - just strike their target and detonate over some huge distance at speeds that would evaporate them with the heat generated by friction (literally).
They posses no engines and FTL does not function in a dense atmosphere or area with complex magnetic/ionic action such as a planetoid - they become glorified FTL rocket bombs unable to effectively guide themselves.
A stray round like this striking a planet though at any value of C would be disastrous, continents and mountains re-written in strikes, burning cities to the ground with the plasma left by the super-heated air.
The dangers of friendly fire when defending your home would be truly insane. They should be sanctioned for use only away from home territory at those speeds.