Thought I'd post an update. Performance is better now, playing in 1440p with FSR2 on Balanced, graphic settings on Medium. I'm getting around 45FPS.
I'm not quite sure what the fix was in the end, but I've installed proton-ge-custom-bin from the AUR, so I've got Proton GE installed onto my system (not just within Heroic). Both the AUR and Heroic are on Proton-GE 9.7. I then added the AlanWake2.exe file to Steam, and forced compatibility to use Proton GE. So whatever extra was happening with Heroic was taken away.
I tried changing my Mesa drivers, but it appears Manjaro is even with the nonfree repo at this point, so I'm still on Mesa 24.1.1, only on the Manjaro-approved version, not the mainline Arch version.
It appears that somewhere within that combination of (1) running the game directly from Steam, (2) using the system-installed Proton-GE package, and (3) reverting to the Manjaro Mesa package the framerate got bumped from lower 20-s to mid-40s. CPU and GPU usage still hover around the 40%, so I feel there would be more improvement to be had perhaps?
I tested the game on my second PC, which is running ChimeraOS, has a 5800X3D chip, 32Gb RAM and a 7600 XT card, where it's easily hitting these framerates and better (but with higher GPU usage, hardly any CPU usage, that 5800X3D is a beast). So I wonder if my 10700K might be a bit of a bottleneck? ChimeraOS' version of Mesa is on 23.2 so maybe there are just some kinks to be worked out in the 24.1 branch?
Anyway.... long story short: I'm happy with the overall performance at this point. Happy to dive into the Overlap and deal with Nightingale!
It's not a patient game yet, but I caved and got Alan Wake 2 the other week. In the early stages so far (the game's quite scary!) but loving it.