It was the same for me. One day it started with 9GB written over night and then it came back again and again and wrote heavily to disk.
It even wrote to disk on each page visit. On some pages it wrote > 70MB per page reload(!).
https://www.macrumors.com/ is such a site. Just navigate around in the forum and you saw the disk writes by SafariBookmarksSyncAgent grow.
I then found out the SafariBookmarksSyncAgent wrote big blobs of data to some tables in /Users//Library/Safari/CloudTabs.db even though i disabled the iCloud sync.
I locked the CloudTabs.db File, killed the SafariBookmarksSyncAgent and since then it never wrote data to disk.
When I saw this thread I came back to take a look if it still writes this much data if I unlock the file again, but it seems like the last macOS update removed the lock on the file and the SafariBookmarksSyncAgent still writes 0 bytes to disk.
Maaaaybe the "disable" option for the Safari Sync in the iCloud settings was kind of defect before macOS 12.4 and they secretly fixed it with the last update.
However, for me the SafariBookmarksSyncAgent does not do any heavy disk writes anymore.
EDIT:
Okay, i found it:
It's /Library/Containers/com.apple.Safari/Data/Library/Safari/SafariTabs.db and the /Library/Containers/com.apple.Safari/Data/Library/Safari Folder, where it's located, was still locked.
As soon as I unlock the Safari Folder where this .db File is in, SafariBookmarksSyncAgent again starts to write to disk like crazy on each page reload into this file.
So I keep this folder locked and yep, the problem is still there.
