damn, I was thinking on that these days: a way to create a little NAS solution based on Raspberry Pi , but with some SSD/HDD disks attached, not just the SD card XD (however I find the SD cards quite reliable so far in my phone, I would need to try a Raspberry for myself )
Your phone should use the SD card as a WORM drive, which shouldn't cause too much wear. The Pi can't do that if it's the only storage medium present. Still, back up your stuff, just because it's there now doesn't mean it can't be gone in an instant, flash storage usually does in an instant without warning
I had an Odroid running a NAS (WD RED drive) configured to do everything in memory instead of writing to the SD card and it still died after a few months.