For whatever reason, I prefer the non-graphical installation process, but I can’t get wpa_supplicant working (perhaps due to hardware/drivers), and the minimal ISO doesn’t have NetworkManager.
Is there a non-graphical ISO available that does have NetworkManager?
No. But you can make one yourself.
https://nixos.wiki/wiki/Creating_a_NixOS_live_CD
You might need to adjust some pathes. I am not sure how current this is.
Also, the “graphical” ISOs are not really changing the process, they just give you access to the documentation and gparted, while the installation process remains the same, but is done in a terminal emulator.
And of course you can always switch to a TTY after setting up the network in the graphical NM if you really prefer the hardcore experience 
Cool, I assumed creating a custom ISO would be a pretty involved process, but it looks straightforward. Thanks!