Im also on Ublue right now, used the ‘make your own’ guide to craft a Hyprland flavoured system. Which basically just means creating a github repo where you put the desired packages in a yaml file then github builds the immutable image for you (on top of the work ublue devs do ofc). Would highly recommend Ublue.
With great power comes a steep learning curve.
This seems like it directly uses nftables rules, but ive seen a couple of aspiring Nix router repos that already offer nix module configuration like https://github.com/chayleaf/nixos-router that dont require you to be a networking expert, however it seems to be a long way off from a featureful router system. If you are after a declaratively configured router i would just go with VyOS.