I ended up having to "delete messages from icloud" on all devices, escalate to support after they weren't deleted 45 days later (the UI shows -15 days in that scenario), reenable sync and then see it still failing to complete a Sync Now, redisable sync and re-delete messages in cloud, wait another 45 days, and then ask them to escalate to the iCloud backend team to run a purge on their end of just my messages in icloud records. It worked, but I don't know if they've had enough time for whatever corruption they found to be patched into their automated ops-repair/cleanup processes (and they wouldn't tell me if I asked), it's only been a couple months.
