What is the best practice to change variables in automations

I am trying to create auto install Ubuntu OS with packer on Hyper-V and I have project like this:

packer-main/
|
|--->http/
|       |--->user-data
|
|--->templates/
|        |--->build.pkr.hcl
|
|--->variables
|         |--->pkrvars.hcl   
      

content in user-data

#cloud-config
autoinstall:
  version: 1
  identity:
   hostname: jammy-daily
   username: ubuntu

content of pkrvars.hcl

hostname = "jammy-daily"
username = "ubuntu"

and build.pkr.hcl

...

ssh_username = "${var.username}"
vm_name = "${var.hostname}"

...

My challenge lies in the need to dynamically modify variables within the user-data before initiating the build process. While I am aware that PowerShell scripts or Bash scripts can serve this purpose, I am curious if there are specialized tools available for streamlining such automations.

I am currently working through something very similar. I found a feature in Packer called templatefile so you can put variables inside your user-data file like so:

#cloud-config
autoinstall:
  version: 1
  identity:
   hostname: ${var.hostname}
   username: ${var.build_username}

Those variables can be passed in through var files or at run time.

Then in your build.pkr.hcl file under source you refer to your template file with http_content, like so:

source "test" "example" {    
    http_content = {
        "/meta-data" = file("${path.root}/http/meta-data")
        "/user-data" = templatefile("${path.root}/http/user-data.pkrtpl.hcl", { var = var, local = local })
    }
}

Keep in mind that http_directory can't be defined at the same time as http_content.