With Niri (and other wlroots based compositors), a lot of people prefer using "greetd" instead of SDDM.
If youre on Noctalia v5, they added "greetd" integration - https://docs.noctalia.dev/v5/greeter/
If youre not - https://wiki.archlinux.org/title/Greetd
I use Niri on NixOS with DMS (waiting till Noctalia v5 to be out of beta before trying it) and made the switch from sddm to greetd, and im pretty happy with it.