What does macOS consider "enough space"?
It's based on the available storage space of your device.  iCloud is a syncing system which means it allows you access to your files from all devices signed-in with the same Apple ID.  When you change a file on your Mac, your iPhone will show you that changed file (after the change syncs, of course). You're not changing a copy of a file, you're changing the actual file. So files are kept on your computer and are stored in Apple's Cloud.
This is different than a system like Dropbox in that Dropbox keeps a copy of a file online.  You can have a 64GB iPhone and 1TB in Dropbox space, there's no correlation between the two.
Taking a sidetrack to understand iCloud better, a good explanation is provided by The Mac Observer:

iCloud syncs your data between your Apple devices. If you delete a file in iCloud on one device, it gets deleted on all your other devices. But that means if one device is infected with malware, and your files get messed up, they will be messed up on every iCloud-enabled Apple device.
iCloud Backup is a good start but it’s not a true backup either. This service does back up many things on your device, but it doesn’t back up data that is already in iCloud. Services like Contacts, calendars, Safari bookmarks, Apple Notes, and Messages in iCloud are not included in iCloud Backup. A true backup would back up everything.

So when you ask &quot;At what point will the Mac start offloading things to iCloud?&quot;, the answer is that it always offloads some things to iCloud. You can fine-tune your storage needs vs speed in accessing files.
On your Mac when you select &quot;Optimize Storage&quot;, it will move things like movies and TV shows that you've already watched from local iTunes storage to iCloud (freeing up some of your drive space).  You also have options to store other items, like email attachments, in iCloud.