I have used darktable, but doesn’t seem to fill your need as it is more a lightroom replacement than Photoshop https://www.darktable.org/

I’m not 100% sure but this might also depend which OS you are running on your pi. Did change from SD card to SSD few years back for my home assistant setup (now running with a mini pc) at the time I went for a geekpi adapter (can’t recall exact model).

Try a quick ducduck search “sata adapter #yourOS#” and see if a list come up. Here is one for HA (but surely more models are compatible than the ones listed here) https://community.home-assistant.io/t/working-usb-enclosures-and-adapters-with-hassos/212763

Edit: as for SSD, I went for Samsung EVO model

Sidenote: all SSD will be more reliable than a sd card because they are less sensitive to rewrite counts

I don’t see why you think those are web based, usually people run this at home but glad you find what you are looking for

As far as photo librabry, you can compare the different solutions at https://github.com/meichthys/foss_photo_libraries

Relying on 4 nvme drive for low power isn’t ideal as nvme drive needs about twice more power compare to sata SSD.

On your Xiaomi first time you launched app, you can determine wether you always allow or allow on prompt (but you should still be able to manage that setting afterwards). You also need to disable battery optimization for that app and set app to be able to auto launch in background. This is at least what I do with my home assistant app to determine GPS location in background (app record every 15mn) and no issues with battery draining.

Don’t have particular case to recommend as the one I have has been printed but get one where you can add an SSD. SD card are not well suited for home assistant due to the amount of overwriting.