(This thread is not for technical discussions to install Nextcloud to Android devices)
People I worked with usually don’t have their computers 
replaced as frequent as their mobile devices. One may buy a new computer every 5-7 years or buy new hardware components to make their old computer satisfy their growing needs, but usually one can have a new phone 
after like 2-4 years.
Repurposing old-but-still-usable android phones is a great idea, since even phone models sold like 5 years ago have a good hardware (aarch64 processors, 3GB+ RAM and 64GB+ Storage) for running Nextcloud servers, and it is, in my opinion, more sustainable than purchasing SBCs just to setup a server on it. And phones obviously uses a LOT less power than old computer. 
Though there is indeed caveats. Most phones don’t accept TF cards as a mean of storage expansion anymore, and though the RAM is large enough, android system occupies at least 1GB of RAM on boot. 
I just hope android device manufacturer can really allows users to unlock it, and opensource their kernels & dtbs so people can port 3rd party ROMs (or even maybe mobile linux like Mobian) to it, to allows users to exploit their devices’ full potential. Many android manufacturers today locked down their devices just to stop users from rooting their devices and to keep their shitty ads on their devices, how pathetic. 