In a hybrid environment, it is suggested to use Edge for Exchange online, because:
About detailed information about Edge server function in hybrid, you could have a look about this article: Edge Transport servers with hybrid deployments
As you said there doesn't exist mailbox on Exchange on-premises, so you could point MX record to Exchange online and disable centralized mail transport. In this way, mail flow will not be through Exchange on-premises, you will not need to deploy Edge server.
Whether we need to deploy Edge is decided by whether you need to send emails through Exchange on-premises. Transport routing in Exchange hybrid deployments
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.