Hi MoonJ.,
I have already done those things and more, but to to avail. Across this community, I am seeing several other people with similar or the same problems with El Capitan, also following similar advice, seemingly to no avail, even though the iCloud support information document makes it clear that Support for iCloud Drive goes back to Yosemite. Is this a system-wide support failure for El Capitan on iCloud? I spent an hour with Apple Phone Support on Saturday as well. This iMac is fully capable of syncing, it just doesn't, for no particular reason. When i go into terminal and enter brctl log -w it shows an error in sync down.
The text is
[ERROR] 0.000 [2018-02-27 12:33:56.627] cloudkit.operation.callback sync.down BRCSyncDownOperation.m:465
can't create zone or register pushes: <CKError 0x7fe779e5e5f0: "Service Unavailable" (6/2030); server message = "cross-zone migration already done"; Retry after 86400.0 seconds; uuid = 0A97FECD-685A-4969-95EF-BDC681EF44A4; container ID = "com.apple.clouddocs">
[ERROR] 2.006 [2018-02-27 12:33:58.633] cloudkit.operation.callback sync.down BRCSyncDownOperation.m:465
can't create zone or register pushes: <CKError 0x7fe779d24ff0: "Service Unavailable" (6/2030); server message = "cross-zone migration already done"; Retry after 86400.0 seconds; uuid = 0972459C-F56A-4C5B-A38E-0B648F3AAB8A; container ID = "com.apple.clouddocs">
[ERROR] 2.014 [2018-02-27 12:33:58.641] cloudkit.operation.callback sync.down BRCSyncDownOperation.m:465
can't create zone or register pushes: <CKError 0x7fe779d91860: "Service Unavailable" (6/2030); server message = "cross-zone migration already done"; Retry after 86400.0 seconds; uuid = C32EDC71-319B-41B0-9A8E-8895C0DCC95C; container ID = "com.apple.clouddocs">
[ERROR] 2.047 [2018-02-27 12:33:58.675] cloudkit.operation.callback sync.down BRCSyncDownOperation.m:465
can't create zone or register pushes: <CKError 0x7fe779d9e1f0: "Service Unavailable" (6/2030); server message = "cross-zone migration already done"; Retry after 86400.0 seconds; uuid = 3E961554-A2EB-4DFB-BCEC-B96FFE8FD4CA; container ID = "com.apple.clouddocs">
[ERROR] 2.125 [2018-02-27 12:33:58.752] cloudkit.operation.callback sync.down BRCSyncDownOperation.m:465
can't create zone or register pushes: <CKError 0x7fe779e11c90: "Service Unavailable" (6/2030); server message = "cross-zone migration already done"; Retry after 86400.0 seconds; uuid = 0A56204D-B82D-41CA-AD4C-15575D0663F8; container ID = "com.apple.clouddocs">
[ERROR] 2.125 [2018-02-27 12:33:58.752] cloudkit.operation.callback sync.down BRCSyncDownOperation.m:465
can't create zone or register pushes: <CKError 0x7fe779c34850: "Service Unavailable" (6/2030); server message = "cross-zone migration already done"; Retry after 86400.0 seconds; uuid = 7300F4B6-EC26-4454-8968-75BC0BA26532; container ID = "com.apple.clouddocs">
[ERROR] 3.990 [2018-02-27 12:34:00.617] cloudkit.operation.callback sync.down BRCSyncDownOperation.m:465
can't create zone or register pushes: <CKError 0x7fe779e1e900: "Service Unavailable" (6/2030); server message = "cross-zone migration already done"; Retry after 86400.0 seconds; uuid = B09ED23D-1668-46D0-8182-300F0C607C43; container ID = "com.apple.clouddocs">
[ERROR] 5.778 [2018-02-27 12:34:02.406] cloudkit.operation.callback sync.down BRCSyncDownOperation.m:465
can't create zone or register pushes: <CKError 0x7fe779e281d0: "Service Unavailable" (6/2030); server message = "cross-zone migration already done"; Retry after 86400.0 seconds; uuid = AA83CE9F-D67C-4D6B-8B2B-59A5D6AC3C09; container ID = "com.apple.clouddocs">
[ERROR] 5.780 [2018-02-27 12:34:02.407] cloudkit.operation.callback sync.down BRCSyncDownOperation.m:465
can't create zone or register pushes: <CKError 0x7fe779dd2310: "Service Unavailable" (6/2030); server message = "cross-zone migration already done"; Retry after 86400.0 seconds; uuid = 55A405A9-8E5E-4C78-B366-1662842BB545; container ID = "com.apple.clouddocs">
[ERROR] 5.785 [2018-02-27 12:34:02.412] cloudkit.operation.callback sync.down BRCSyncDownOperation.m:465
can't create zone or register pushes: <CKError 0x7fe779d37cb0: "Service Unavailable" (6/2030); server message = "cross-zone migration already done"; Retry after 86400.0 seconds; uuid = FE156981-A419-49AF-A632-3D335A08E622; container ID = "com.apple.clouddocs">