The database for your Vault and password stored in Vaultwarden will be encrypted by the Bitwarden clients prior to being sent to your self-hosted instance.
There are a few things such as possibly your config.json and other Bitwarden item that may have information in plain text, so I would say it never hurts to double encrypt information prior to send.
Zero trust environment are becoming more common place in today’s age, and IMO any important data at rest should always be encrypted.