Nix in single user mode can apparently work with SELinux in enforcing mode, although AFAIK binaries installed via Nix can not have SELinux metadata which could be an issue for some programs.

Determinate Nix claims to have seamless integration with SELinux (unlike upstream Nix and Lix, but it’s not a fork, alright whatever you say Eelco). Using that and removing the proprietary garbage their installer also adds might be easier than making regular Nix play nice with it.

My understanding is that most of this is down to ARM’s (relative) lack of standardization. Consumer ARM SoCs don’t even have ACPI, so you aren’t even guaranteed to be able to do things like powering off the system. Qualcomm pretty much has to add some minimum support for their SoCs to the kernel because most of their consumers will want to get Android working on them, but that doesn’t mean they’ll do more than they have to for that.

There’s a reason you can install Linux on any x86 PC and it will mostly work, but you can’t install an ARM Linux on a phone. Even Android forks like Lineage don’t support all Android phones, even though they’re shipping basically the same thing the manufacturers are.