Not a bad idea... would be pretty cool and phtn.app is much more accessible than photon.xylight.dev.
I guess a related question is what is your vision for how users would typically use Photon. Do you want to host a single/main application that many users end up using or would you rather have instances (or individuals) deploy their own applications (ie. nu.lemdro.id)? or possibly both?
If the former, then phtn.app makes sense. Otherwise, just photon.xylight.dev is fine since it would sort of encourage others to deploy their own.
Another thought: if you follow through on making Photon into a PWA, then phtn.app would probably fit into that branding, more so than photon.xylight.dev.