Even though I am not nearly close to 1000’s of units yet, I am faced with the same challenges. I support what @armor and @ScruffR said, they make good points.
From my perspective (as a newbie) I do not really see the need to upgrade to P1 just yet. I have multiple Photons in commercial and light industrial environments and they seem to do just fine, with the added bonus of being able to easily swop them out should something go wrong. I mount them on custom designed PCB’s (in headers or surface mounted depending on the profile) which eliminated the need of any wires and connections which already is a huge improvement for me.
I am not sure whether there are any downsides to using Photon and Argon modules instead of production boards, maybe someone more familiar with Particle can assist with that.
Due to the huge difference in our exchange rate vs US$, the cost per device is a huge challenge here as well. As @peekay123 mentioned, if you are looking at possible 1000’s of units I would most certainly recommend speaking to Particle sales team, well, at least, this will be my approach if our prototypes pass the test and we need to go into production 
Regards,
Friedl.