How do I isolate the amount of a recurring contribution in CiviRules?
You can use search builder using condition shown in image to create a smart group.
You can add more filter based on your payment processor to limit the result to first payment, for that you will need to diff between first payment and second payment in civicrm_contribution table. For example if you using paypal for recurring contribution than the first contribution has invoice_id set but the subsequent ones are set to NULL so you can use filter like Invoice reference not null, for Smart debit you can use contribution source eg contribution source like %Membership form%.