This question is fantastic. Just want to throw in some random thoughts. Not an native speaker though - hope you'll understand most of it anyways.
My preconceptions
The goal of psychological warfare is the weakening of the enemies economic capacities, their fighting performance or their leaders ability to assert appropriate decisions through means of behavioural modification of the target species.
Psychological warfare on a whole race that is as varied in physiology, function and probably in smaller scope even philosophy as the Umiak is an intricate matter in itself. Targets for psychological warfare for example might include their command structures and processes, their fighting forces, their productive population or their reproductive facilities.
Targeting secondaries
War is generally not a rewarding process in itself. Leading a population of that size and diversity to strife for a war requires a strong dedication of the working population for the fighting forces. Umiak are granted that perk through their relatively weak population individuality regarding their "greater good", their focus on mastering their craft on a very basic level and their flexibility to exhaust their own existence. Giving up your own life for an abstract high level achievement is not a mechanic that can easily trained by evolution, therefore it's save to assume that Umiak include other races as levy troops not out of evil but out of neccessity.
As the universe just would be very unlikely to spawn another race with psychological traits compatible with Umiak philosophy we just have found an application for our psychological warfare. A viable long term strategy would be to target the Umiaks bonds with their lateral races, cutting them off from most of technical innovation and or non-umiak bio diversity which might be beneficial in any biological warfare scenarios.
This could be achieved by just increasing the communication potential of all involved races. The more the general populus knows, the easier it gets to motivate and create resistance.
Inverted psychological warfare.
A common problem in warfare is to get your armed entities to actually engage the enemy. The time you take a shot you will stick your head out of your trench and open up a possibility for the enemy to stab you in the face really really hard.
So basicly it's never a good idea for anyone to make the first move. An army of willingly brave soldiers will always outperform an army of onvoluntary brave soldiers. So you want to target your fighting force and your population with propaganda that uplifts the idea of war and fighting above the idea of normal population habitus. This can and probably already is partially solved by gathering soldiers in enclosed societies or castes where you can safely introduce behavioural modification without endangering your civil society.
The loroi forces depicted in the story are dedicated special forces which probably have been choosen not entirely by skill but by dedication to their skill. The loroi main army groups probably aren't en par with that dedication and therefore could always benefit from further indoctrination.
Copy and paste umiak mentality into your forces.
As depicted umiak are bound to differentiate more often in quantity rather than in individuality or quality. Therefore an army that will psychologically influence umiak authorities should be build according to the enemies understanding of force. Depending on the impact of psychological reasoning in umiak warfare it might make sense to design cheaper larger ships with larger weaponry to gain the ability to actually deflect an impending umiak attack by show of force only.
As vaguely hinted in the story, loroi main forces are not as performant at holding line operations and umiak seldom see the need to cancel offensive maneuvers because of the presence of loroi fleets.
Bottom line is: if you can't communicate your strength to the enemy, you loose an opportunity to play your cards right.
In summary i would say there is always room for psychological warfare, but it's not the primary means of fighting a race that enjoys the benefit of having close to no individuality in it's fighting ranks and furthermore can rely on biological engineering. Which implies nearly unlimited and cheap behavioural modification for it's own personel.
Hope the story doesn' go from awesome into cliché anytime soon. But i fear the umiaks greatest lethal weakness will probably be a plot device which should be introduced near chapter 4 to 5.