Long story short, iPhone Pro and noir is great. No third party apps— just the default iPhone Pro shooting in Noir filter mode, and later with some processing in the built-in Photos app:
My first verdict:
iPhone Pro is truly “good enough” in terms of image quality, aesthetics, for artistic photography.
It can be used as your solo camera (if you desire).
iPhone Pro has TRULY GREAT in-camera processing tools (Apple Photos is phenomenal in terms of the capabilities).
Don’t use third-party apps, just use the default apps.
Why? “It just works”. Also better integration with the whole Apple ecosystem.
The photos look very good. Not as good as RICOH GR III, but still very good!
4. Downsides of iPhone Pro
iPhone Pro with noir is great, but if you want the apex of monochrome, better to use RICOH GR II, RICOH GR III, or shoot black and white film.
For example, RICOH GR III default high contrast black and white JPEG mode gives you far “deeper blacks”, and far more dynamic range: