A networkd-based backend for networking.wireguard was merged recently. It will be enabled if you enable networking.useNetworkd or networking.wireguard.useNetworkd.
You must make sure that your secrets are readable by the systemd-network user. Otherwise, systemd-networkd will silently fail while trying to set up your WireGuard interface.
PR: nixos/wireguard-networkd: init by Majiir · Pull Request #259092 · NixOS/nixpkgs · GitHub