I use ZFS on top of LUKS with the same three-way root/nix/home breakout and a separate SWAP partition.

Works pretty well and sidesteps the partition-sizing issue entirely, since the pools don’t have to be a fixed size.

I suppose you are aiming for impermanence? 50GB seems good enough if you're not installing too many tools. If you haven't already, take a look at nix.* options because they have some storage optimisation options. That and CLI tool called nh, which cleans stuff up for you.

{
  nix = {
    settings = {
      auto-optimise-store = true;
    };
    gc = {
      automatic = true;
      dates = "daily";
      options = "--delete-older-than 10d";
    };
    optimise = {
      automatic = true;
      dates = [ "weekly" ];
    };
  };
}