So, a late reply, because I wanted to think about this a bit.
If you take the “metaphysical” position of DE as being presented mostly via the metaphor of the Pale/Hollow , the conversation with the Phasmid, and the general tone of the writing regarding both different ethical/philosophical positions, and how people’s lives generally go…
I felt that the point that the Phasmid conversation made was that humans are unique in that they actually can choose to do things or not - they “add intent / meaning / context” to the universe, which other things don’t do. The Phasmid can’t choose not to exert its psychic influence on the Old Soldier, even though it is increasingly killing him; it’s something the Phasmid just does, because it is in its nature. And the reason why the other creatures are so… existentially terrified of humanity… is that humans do choose - they pick up all these influences from the world, and then impose meaning, morality etc [and make choices, or believe they do] as a result. (I think that Hollows, and the Pale around them, are a reaction against the structuring of reality by humans adding “meaning” and “observing” reality [in the Berkelian sense, not the QM sense]; they’re the other side of a metaphysical see-saw, places with zero-structure and meaning.).
The Phasmid (and all of nature excluding humans [and other conscious entities presumably]) isn’t “good” or “bad” (or “evil”), because morality is a thing that people create, it’s not inherent to reality. Its advice to Harry re his feelings for his Ex are the only advice that a non-conscious being could give about something which causes “pain”; filtered through your Harry’s personal philosophical biases [which presumably colour his Inland Empire “translating”].
So far, so every post-Schopenhauer field of philosophy, so this could be anything vaguely post-modernist or existentialist in position.
I picked up on an Absurdist, specifically, tone roughly following your reasoning: DE is cynical about all positions and won’t let you pick one without pointing out both the positives and huge negatives - but it’s particularly upset about centrism and moralism, which are the closest things to a kind of philosophical nihilism [in the Nietzchean tradition, at least] in the setting. DE seems to want you to choose to believe something, in the knowledge that what you’re devoting yourself to is, necessarily both inherently flawed, and also not “true” in a deeper metaphysical sense. The value is in both - wholehearted commitment to adding meaning, whilst simultaneously being aware of the inevitable futility of this on the universal scale. The striving is what’s important.
(I think the inevitability of the Pale destroying all existence serves as a pretty blunt reification of the inevitable futility of all human endeavour, on the long scale; especially as the Pale explicitly erases even meaning, the one thing that Existentialism thinks will “outlive” you.)
And, really, the central murder mystery itself is a pretty Absurd activity (as is the work of the RCM in general): we’re trying to do a “bit of good” and solve a problem, but in the end, the killer is questionably responsible [being not of sound mind], we’ve tried to defuse a massacre, and had mixed success… and we know that there’s going to be more things to investigate in the future.
(Even Harry’s own unresolved psychological issues are Absurd in this sense: he obviously strives to be a good cop, keep things in control [and if you play him as I did, can have every intent to “keep it all together for good this time”], but his colleagues from his Station make it clear that they expect this to be just like the last N times this happened, and that his eventual failure is inevitable. A hero with a Sisyphean struggle, played during the upswing, is almost a perfect Absurdist hero.)