Apply a promition only on a product variation
You need to implement you own promotion condition plugin. Theres a condition plugin defined in commerce_product module in Commerce module since 2.0 release but I recommend you to use latest stable release. Check this class Drupal\commerce_product\Plugin\Commerce\Condition\OrderItemVariationType thus it only checks for variation type (aka bundle), you can easily write your own checking specific product variation instance. Also worth checking OrderItemProduct condition plugin I think you need a mix from both. This plugin is located in the same namespace than the first one.

Update

There's a feature request for that see this link. Thanks to @bojanz!