Precursors currently fill the role of pirates. It would be tricky to set up the economics necessary for believable pirates, but I could add other races that function with a 'raiding' mentality.
I always though the other spoiler race was alot more fitting for a "pirate" like role.
I mean for these your already half way there with how they reclaim the wrecks of destroyed ships to grow their own numbers, right?
All that's missing is some raiding behavior so they send out ships to nearby systems and prey on commercial ships trying to destroy them and salvage the resources before you can arrive in strength. If ability to tow wrecks was introduced they could even tow home the wrecks.
Being sneaky like that and hit your weaker slower ships while avoiding military ships where possible, or luring them into traps also would fit better with their theme and weaponry I think.
The role of Precursors I always considered to be one more as gatekeepers/guardians of bodies or systems with desirable minerals, anomalies, ruins or colony sites.
Another way to promote raiding more might be to make it easier to sneak past jump points. Maybe make a Jumpdrive option for a raiding ships which makes the jumpdrive 3 times as large (prohibitive for warships) but gives it say 100 times the jump radius and limits it to self jump only?
The huge downside is that they are much more vulnerable, a single ship with an energy weapon could cruise through and obliterate them.
Unlike an undefended fresh fringe colony with some terraforming installations that will fall to a single enemy ground battalion, and even be captured intact by them or what?
The ships are less vulnerable in fact since they have the theoretical ability to try to run away given some warning, while the colony most certainly can't.
If you want to keep either installations or ships safe you need to protect them, that's not something that is inherently a weakness of either of them IMO.