I've never heard of kexec before; not very familiar why installing NixOS using kexec is only supported on x86 but not aarch64.
I wonder what are the advantages and downsides of nixos-anywhere v.s. nixos-infect.
I would not use it for a production system. Currently trying to migrate off of it to, I dunno, something else and Ansible. Nix has eaten hours of my life and upstream packages break all of the time. Couple of weekends ago the Mullvad module broke. Before that Virtualbox. Before that ZFS. You can have a perfectly fine configuration that you never touch but upstream instability will prevent a nixos rebuild from actually working.
Basically nixpkgs' maintenance policy is that only one version of a package should exist in their repositories. This being intended to reduce maintenance overhead since the nix package manager can freely switch package versions based on the nix channel (read: git branch or commit that nixpkgs is on).
The problem is that unless you meticulously start version pinning all your dependencies, your tooling will always run on the absolute latest version available, regardless of major/minor updates on either stable or unstable. Which can obviously cause problems.
It requires an extra step of care, one not particularly helped by the fact that actual version pinning for an individual package is done by the nixpkgs channel, rather than specifying the desired semver spec so unless you know beforehand what version is at which commit, it probably won't be helpful when it ends up rendering the OS unbootable (not to mention the questionable use in manually typing over git commit hashes but I digress on that).
For grabbing a package from another version of nixpkgs, its quite straightforward to only use that specific nixpkgs for one package and nothing else. NixOS also keeps old generations around in case the new configurations break anything, so I'm not very worried about the OS being rendered unbootable in any case (although I've never managed to break my NixOS, so I can't attest to having made use of this personally).
Not only is this now way cheaper (which increases profit margins for paid services I run), but it's also significantly more reliable as a one-person developer and de facto SRE.