The trade guild is likely the first NPC faction players will encounter and learn from. It makes sense to introduce a common and necessary defensive system to players early on.
I think a good way to introduce PD would be to have very few TG ships in core space with PD- it's a personal expense that a freighter captain makes themselves, rather than something the TG will cover. This gives an opportunity to introduce PD without breaking the lore of "TG doesn't part with their money easily."
Then, mid-space has nearly every freighter having at least a PD turret or two, and some even have the occasional escort.
Finally, in the nearly lawless area of the galaxy (arms, very rim of the center section) has freighters with a few small offensive turrets, lots of PD, and nearly always have escorts. Freighters in this area could even have upgraded armor or shields.
Criss , have you considered opening up opportunities for the community to work with the devs on this sort of thing?