You will need to migrate, there is no hybrid model.
https://learn.microsoft.com/en-us/azure/devops/migrate/migration-overview?view=azure-devops
If you truly need a hybrid Cloud + On-Premises approach then you should stay with Azure DevOps Server, and run a multi-server configuration that spans from on-premises onto Virtual Machines (not Azure DevOps SaaS) in Azure. https://learn.microsoft.com/en-us/azure/devops/server/install/multiple-server?view=azure-devops-2022