While technically this sounds interesting, using an old phone to install ONE App and call it a NAS is not what I expected.
Additionally, a storage solution (NAS) relying on a single eMMC or microSD neither seems very reliable nor fast enough.
Considering the small size and the lack of connectivity of "an old phone", there are way better solutions out there in my opinion - even if you'd just like to use it as a "vacation device" in a hotel.
I personally use a GL.iNet Beryl with OpenWRT, which is about 50 bucks used, but there is also stuff from FriendlyElec (NanoPI NAS Kit).