Hmm so if planet/colony does have only 1 ATKSPD, then you really would need to build up mine fields and orbitals early in game to defend vs players. Otherwise a planet can only kill 1 ship per round.
I'd suggest to maybe making ATKSPD for planets = population / 20mil = 20 ATKSPD (for 400mil pop planet)
But planet killers I really not even sure about those. But knowing the sheer coast of the unit, I would want to see it with low initiative but rather high ATKSPD.
Just think back to Death Star from Star Wars. It had 1 main weapon to kill planets (check), but it also had a ton of surface mounted cannons and carried a bunch of fighter ships.....
So, unless you guys figure out a way for planet killer to spit out unlimited amount (but no more than 10 at the time) of fighter ships. Thank I'd advise to set base ATKSPD to ~10