I have similar idea of using my old Xiaomi Mi A1 as a server, but I have never actually done it. The author's first idea and mine was similar, replacing Android with PostmarketOS. And here is the difference, I was not brave enough to actually install PostmarketOS to my phone. Especially after looking at the Postmarket Wiki where not all features for my phone are flagged as supported.
In the end, I abandoned the idea and opted on purchasing an Android TV Box that has been installed with Raspbian for my home server. Sure it may not be as powerful as my phone, but it came with full blown USB that I can use to add any storage I want and it was cheap (today, it was approximately USD 11-12).
Correction for my previous comment, it is not Raspbian, but Armbian. The Armbian was not pre-installed from the factory, it was an Android TV Box that some tech-savvy guy in my country reinstalled. And in the end, I reinstall the Armbian in a micro-SD card, so the pre-installed OS was not used at all.
In the end, I abandoned the idea and opted on purchasing an Android TV Box that has been installed with Raspbian for my home server. Sure it may not be as powerful as my phone, but it came with full blown USB that I can use to add any storage I want and it was cheap (today, it was approximately USD 11-12).