I have a disks.nix in my git repo, added it to my configuration, and disko-install ... worked perfectly! My my disks config looks like (basically it is the luks-btrfs example in the docs):

{
  disko.devices = {
    disk = {
      main = {
        type = "disk";
        # When using disko-install, we can overwrite this value (device) from the commandline:
        # $ sudo nix run 'github:nix-community/disko/latest#disko-install' -- --write-efi-boot-entries --flake '/tmp/config/etc/nixos#mymachine' --disk main /dev/sda
        # see: https://github.com/nix-community/disko/blob/master/docs/disko-install.md
        device = "/dev/nvme0n1";
        content = {
          type = "gpt";
          partitions = {
            ESP = {
              size = "512M";
              type = "EF00";
              content = {
                type = "filesystem";
                format = "vfat";
                mountpoint = "/boot";
                mountOptions = [ "umask=0077" ];
              };
            };
            luks = {
              size = "100%";
              content = {
                type = "luks";
                name = "crypted";
                # disable settings.keyFile if you want to use interactive password entry
                #passwordFile = "/tmp/secret.key"; # Interactive
                settings = {
                  allowDiscards = true;
                  # keyFile = "/tmp/secret.key";
                };
                # additionalKeyFiles = [ "/tmp/additionalSecret.key" ];
                content = {
                  type = "btrfs";
                  extraArgs = [ "-f" ];
                  subvolumes = {
                    "/root" = {
                      mountpoint = "/";
                      mountOptions = [ "compress=zstd" "noatime" ];
                    };
                    "/home" = {
                      mountpoint = "/home";
                      mountOptions = [ "compress=zstd" "noatime" ];
                    };
                    "/nix" = {
                      mountpoint = "/nix";
                      mountOptions = [ "compress=zstd" "noatime" ];
                    };
                    "/swap" = {
                      mountpoint = "/.swapvol";
                      swap.swapfile.size = "8G";
                    };
                  };
                };
              };
            };
          };
        };
      };
    };
  };
}