Hello Mati,
Thank you for your feedback.
I assure you that Google for WooCommerce has all the features that a modern plugin for Google Merchant Center should have.
In this message, I shared a solution with you that should work fine in your case.
This documentation mentions that:
Currently, the Google for WooCommerce extension doesn’t support showing all the custom fields available in a shop when mapping attributes. This is to avoid noise due to the huge quantity of Custom Fields in a shop. However, custom fields support can be enabled for attribute mapping using a code snippet.
Since this involves custom coding we cannot help you with that as per our support policy.
At this point, consulting a professional developer is the best way forward.
Best regards.