Messages sync to iCloud- not freeing up local space on iPhone

Hello-

I have a 128GB iPhone 12 running the latest iOS. I also have a 2TB iCloud plan that is maybe 30% utilized, plenty of space.

My iPhone is constantly running at it's Storage limit- and the main culprit is Messages at over 70GB, despite having it sync with iCloud.

From what I understand, when syncing with iCloud, it should be offloading some of the older message content to free up space on my phone, however this doesn't seem to be happening.

I have selected it to sync with iCloud (this was about a month ago)- any my phone is on the charger and successfully completes an iCloud backup every night.

I've tried toggling iCloud syncing for messages on and off with phone restarts in between, but can't seem to get it to budge. Any ideas? It seems that the messages are likely synced up, but my phone isn't interested in making more space.

I'm not interested in deleting old messages as I understand that will remove them everywhere, which seems unnecessary given how much iCloud space I pay for and have available.

It's been suggested that I turn off iCloud syncing for messages, then delete some older messages and then turn syncing back on, but I'm nervous that it will remove them permanently everywhere.

Thoughts? Ideas? Happy to provide any other info. Thanks!