So, this is from a fresh start of the new build. Starts with a URLTrackingQueriesManager error, but everything else looks clean. It synced nicely with the non-RC build, but did not sync with my iPhone.
[2025-05-05 16:50:10] URLTrackingQueriesManager: Error: Failure writing general URL Queries to remove
[2025-05-05 16:50:10] ContentBlockerManager: Successfully updated precompiled advanced content rules
[2025-05-05 16:50:10] CKTabGroupSync: No named window state to restore
[2025-05-05 16:50:10] CloudKit: CloudKit > batchUpdate 6 records, 0 records to delete
[2025-05-05 16:50:10] CloudKit: CloudKit > runBatchUpdate in-progress or no network connection
[2025-05-05 16:50:10] CloudKit: CloudKit > runBatchUpdate in-progress or no network connection
[2025-05-05 16:50:11] CloudKit: fetchLatestChanges > enterSyncProcess
[2025-05-05 16:50:11] CloudKit: Devices - processQueuedSyncedItems > enterSyncProcess
[2025-05-05 16:50:11] CloudKit: TabGroups - processQueuedSyncedItems > enterSyncProcess
[2025-05-05 16:50:11] CloudKit: Tabs - processQueuedSyncedItems > enterSyncProcess
[2025-05-05 16:50:13] CloudKit: TabGroups - processQueuedSyncedItems > leaveSyncProcess
[2025-05-05 16:50:13] CloudKit: Devices - processQueuedSyncedItems > leaveSyncProcess
[2025-05-05 16:50:13] CloudKit: CloudKit > runBatchUpdate in-progress or no network connection
[2025-05-05 16:50:13] CloudKit: Tabs - processQueuedSyncedItems > leaveSyncProcess
[2025-05-05 16:50:14] CloudKit: CloudKit > batchUpdate::success
[2025-05-05 16:50:14] CloudKit: CloudKit > batchUpdate 1 records, 0 records to delete
[2025-05-05 16:50:14] CloudKit: CloudKit > batchUpdate::success
[2025-05-05 16:50:23] CloudKit: CloudKit > batchUpdate 1 records, 0 records to delete
[2025-05-05 16:50:23] CloudKit: CloudKit > runBatchUpdate in-progress or no network connection
[2025-05-05 16:50:23] CloudKit: CloudKit > batchUpdate::success
[2025-05-05 16:50:23] CloudKit: CloudKit > batchUpdate 0 records, 29 records to delete
[2025-05-05 16:50:25] CloudKit: CloudKit > batchUpdate::success
[2025-05-05 16:50:26] CloudKit: CloudKit > batchUpdate 1 records, 0 records to delete
[2025-05-05 16:50:26] CloudKit: CloudKit > batchUpdate::success
[2025-05-05 16:50:28] CloudKit: fetchLatestChanges > enterSyncProcess
[2025-05-05 16:50:28] CloudKit: Devices - processQueuedSyncedItems > enterSyncProcess
[2025-05-05 16:50:28] CloudKit: TabGroups - processQueuedSyncedItems > enterSyncProcess
[2025-05-05 16:50:28] CloudKit: Tabs - processQueuedSyncedItems > enterSyncProcess
[2025-05-05 16:50:29] CloudKit: TabGroups - processQueuedSyncedItems > leaveSyncProcess
[2025-05-05 16:50:29] CloudKit: Devices - processQueuedSyncedItems > leaveSyncProcess
[2025-05-05 16:50:29] CloudKit: CloudKit > batchUpdate 1 records, 0 records to delete
[2025-05-05 16:50:29] CloudKit: Tabs - processQueuedSyncedItems > leaveSyncProcess
[2025-05-05 16:50:30] CloudKit: CloudKit > batchUpdate::success