Hi there MagnusNate,
Thank you for using the Apple Support Communities! I understand you have a question about Messages in iCloud and syncing messages between your iPhone and Mac. I am happy to help.
Messages in iCloud is a way to sync your entire message history across all your devices, while saving space on your iPhone too: Keep all your messages in iCloud - Apple Support. If you decide you want to turn this service off, you can still have your messages forward from your iPhone to another device through Continuity, but this will not sync the entire conversation history between devices. It allows you to, instead, continue the conversation from the device you want but will not show you the history of your messages. You will only see messages sent and received from the time Continuity is set up, and after: How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac - Apple Support.
If you are looking to free up iCloud storage space, but keep Messages in iCloud active, take a look at this article for other ways you can manage your iCloud storage: Manage your iCloud storage - Apple Support.
Best regards.