Some existing AI raid behaviors:
1) Siege
2) Sapper
3) Raid and attack to death (ours or theirs)
4) Raid and kidnap/ steal then escape
5) Raid and retreat on significant raid party injury
Many of these might include an "attacking immediately" or a "preparing" modifier at the start
I would propose to break #5 into 2 options:
5a) Raid and retreat on significant raid party injury, full retreat
5b) Raid, retreat, and regroup on significant raid party injury
In 5b have the retreating raid party stop at the edge of the map for a suitable period of time, essentially going back to the "preparing" mode. If the colony does not follow them and ambush/ destroy them... then let the raid party stabilize wounds, etc. and have new reinforcements arrive equal to some RNG, say up to approximately half the original party strength, join the raid after a suitable waiting time for a second assault on the colony.
5b offers the reward of potentially more recruits from raiders that would otherwise escape, it also offers the significant risk of "here they come again!" with a fair bit of apprehension drama and story telling potential.