How to replace the buttons (attachment) only w/ Slack interactive buttons response

You can only replace the complete message, not just a part.

There are two options to update the original message:

1. Respond to the Slack request with {"replace_original": true}

2. Use chat.update

If your original message was not of type ephemeral you will get a copy of the original message as part of the payload from Slack in the original_message property, which can be helpful to update the exchange the original message.

See this page in the Slack documentation as reference.