It's not as wide as you'd been made to believe. Ubuntu Touch does around 50% of what you're listing.
Web browsing works fine, though the older Chromium is showing its age and may not be running with full hardware acceleration. iMessage is a non-starter, unless someone wants to implement a client for AirMessage, which would itself be fairly simple. WhatsApp works through a web-app with some rudimentary push (if I remember correctly), but a native app is not outside possibility. Social media could be done through the browser, several Reddit clients and one for HN exist, and I personally have written a YouTube client (needs to be adjusted for API changes). Photography is possible, though maybe not on Google Camera level. Email clients exist. A navigator app with OpenStreetMap data too exists. As for medical stuff, unless it runs in the browser, it likely won't even run in Waydroid (Android compatibility) because SafetyNet.
The UI itself is very snappy even on older devices and the battery lasts enough of the day, through not as much as on Android. While it may feel a bit locked down with its immutable FS (you can unlock it, but OEM updates will reset root) compared to other mobile Linux distros, you can still do a lot, from running desktop apps through DosBox and Wine, to editing the shell's QML for custom UI, to automating system actions with the shell. If you have a Pixel laying, I highly recommend you at least try it. Things aren't perfect, there's a bit of legacy code that needs people, but it's a good daily driver.