As the title says, I'm kinda looking for a good transport or freighter to pirate with, excluding the Percheron and the Pelican (TheUnforgiven, I know you wanna scream Pelican), since I want to see if there is anything worth using OUTSIDE of those already good transports (cuz RP and diversity)
I would say Serenity. Well, not only to the fact it was used by Space Cowboys (lol), but also it is rather good ship (assaulting it as indie was a VERY fatal mistake every time).
Bumblebee, Celestra, Gull civ or celestra would be the most agile all sub 2k'ers but not much in terms of armor. PTrans still best armor/agility/weapon/core ratio imho. freighters would be really interesting for pirating but really have no how it would work with them. Now smuggling would be another matter though.
I have had quite a good success in ripping apart ships hauling ores and silent when pirated. Sleeker and agile than the PTrans but sacrifices some 300 or 400 cargo space for that agility. btw, Junker ID on it gets 90%. I don't know about pirate ID. Alternately, Taureau is a cool option too. PTrans is a standard choice for cargo piracy