On my Linux laptop, I use bubblewrap sandboxes with their own encrypted filesystems and granular permissions for each of my projects. Very easy and secure with NixOS, surely better than what I could do with an Android device.
You could be dividing up your projects into highly sandboxed environments on Android too. It has support for running multiple hardware accelerated virtual machines running desktop Linux and it wouldn't be that hard to support creating those with NixOS and other distributions instead of only the standard the Debian images provided by Android. That will happen as it gets more mature.
See the explanation at https://news.ycombinator.com/item?id=49364220 for why desktop Linux is nowhere close to AOSP for privacy and security. If you want more details, there's a lot of deeper coverage.
That is fine but obviously we are talking about threat model in comparison to each other.
>Google Play Services, which are needed
They are "needed" if you want to use other closed source apps that rely on play service feature. If you don't want/use closed source apps there is no reason to use Google play services at all, that's why they are optional.
>running with full access to all my data
GrapheneOS sandboxes play services specifically to run with user permission instead of system and can be used in a completely different isolated profile for your other private user data.
NixOS by itself does nothing to increase security outside of the supply chain. There is for example no default application sandboxing, Mandatory access control or hardened memory allocation (software or hardware). Just to name the most basic security feature.