Set files content to anoter file on ExecStartPre stage

Help
1

So, continuing to try ton configure a VPN server from a .nix file, I’m trying to set the configuration file from a immutable file + custom information in a different file.
Let me give some example to try to be clear:

So, the first variable is declared in the let session:
This file is generate using a .tpl file as a template:

let
  Template = nixpkgs.writeText "config-server.conf" (builtins.readFile (builtins.toPath (../templates/server.tpl))); 
  ConfigFile = "/tmp/server.conf";

So, those lines creates two variables, one called Template with the values from the /template/server.tpl file.
Te second created a variable called ConfigFile which contains the path to the configuration file, in this case /tmp/server.conf.

Ok, after that on the “in” session, I have a system.activationScripts that update the value of the file /tmp/server.conf

  system.activationScripts.script.text = ''
    addr=${cidr}
    instanceName="$(cat /etc/ec2-tags/name)"
    rm -f ${ConfigFile}
    if [ $instanceName == instance-1 ]; then
      addr="''${addr/0./1.}"
    elif [ $instanceName == instance-2 ]; then
      addr="''${addr/0./2.}"
    fi
    echo -e "server $addr 255.255.255.0" >> ${ConfigFile}
  '';

So, on the activation stage the value of the ${ConfigFile} (/tmp/server.conf) is updated based on instance name. (this part is working, I checked that the file was updated.)

Then, on the systemd.service ExecStartPre, I want to merge the value of the immutable file ${Template} into the ${ConfigFile} (/tmp/server.conf)

ExecStartPre content:

    ExecStartPre = [
      ''${nixpkgs.bash}/bin/bash -c "${nixpkgs.coreutils}/bin/cat ${Template} >> ${ConfigFile}"''
    ];

However is not working.
After the activation the ${ConfigFile} (/tmp/server.conf) have only the value set on the activation script and the value of the ${Template} was not merged.

Any hint where Im mistaking ???