There's also postmarketOS, which is more like a 'normal' Linux distro, so probably a better choice if you want to use the phone as some sort of compute node.
What's beautiful about pmos is that you can use docker on it. Very easily you can set up a very functional home lab/server off an old phone.
I did so with an old OnePlus 5
Big if though. Each device requires special support from the community, and even big name devices don't always make it.
Plus some providers mandate that bootloaders can't be unlocked if sold bundled with their service, even if that device is normally unlock-friendly.
Not for server workloads though.
Docker is still a PITA to run on android, needing a custom kernel and also a patched version of docker