Yes. Niri has been packaged for NixOS, so you’re overcomplicating it: programs.niri.enable.
The instructions from their repository are for if you want the latest of the latest version, ignoring software releases and just straight building whatever is on main. You can do that, but especially for a newbie I’d recommend just using what NixOS has packaged - it’s properly integrated and tested, so there are fewer issues to fall for. Plus, you won’t need to really use flakes, which can be confusing for a newcomer.
You likely don’t even need a reinstall if you just remove what changes you made to install niri so far from your configuration, and if you use a display manager you can have both desktop environments installed and just switch between them with the display manager. gdm should be configured to do that correctly if you use the NixOS options for everything.