Just to consolidate, here are the errors and info I’ve received in the logs:
It works for me on Mac when I set max connections to 4.
Also it fails when I start sync on my 2 devices (but when I close app on iOS, Mac DT start syncing again) .
BTW:
How much space does your iCloud account provide?
I have 50GB plan 
Screenshot 2021-02-10 at 17.52.10.pdf (39.6 KB)
2 TB, with 1.5TB of free space.
Btw, on a positive note… the sync via Cloudkit appears to be much faster, very much appreciated 
The transfer of my 15.1 Gb database just finished. No changes from the default settings. Just had to wait through the rate limit / burst of data cycle until it finished.
Just tried a really small database, my global inbox, with 37 items, 21 Mb. It uploaded immediately, no errors.
Among all those recurring log messages, there’s a new one in between. Don’t know if this helps or is related to this issue, but maybe is useful:
17:49:32: /Users/stephan/Library/Caches/TemporaryItems/DEVONcloudy 95171/Thread 16/46/0f4af0ec558e467b8ffc9429e3ed94a9-c099b47016769dea183c94e3ee4ce8f56c8eb054e48974ed4c09ea493d63c252.177 The file doesn’t exist. (NSCocoaErrorDomain 4)
Sync of DB with 23GB just finished.
FWIW: I’ve uploaded 5 more databases (all < 2 GB) without errors. I have two larger databases left to upload. By watching the DTPO status line and error log, it looks to my eye that the CKErrorDomain 6 errors appear for large files in a database, whereas the CKErrorDomain 7 errors occur appear for large NUMBERs of files. Both have been temporary for me needing no corrective action aside from waiting for DTPO to retry and complete the upload.
I have a total of 6 databases. The first one, which is 4.64 GB uploaded without issue. The second two I’m trying to upload are both under 2 GB and both keep getting the errors. (I am attempting one at a time.)
For me similarly, biggest problems were with databases that contained many pdfs, but finally synced on my mac! now I need sync my other iOS devices
I just finished a support ticket and the 6 and 7 errors occurred within a finite time span. After about half an hour the sync quickly finished as the engine resumed its attempts.
Sync just stops on mine after a while and doesn’t attempt to finish. If I kill DEVONthink, it will start synching again, but then hangs. It’s synching, but there are a lot of gyrations to make it go. After the last restart (about 5 minutes ago), I am at least getting the same errors in the logs letting me know it is at least trying.
Sync doesn’t constantly retry. It should resume on its own within 4-5 minutes. Don’t just kill it. Be patient with it.
I waited for almost 45 minutes with no activity (set it prior to going to lunch).
It seems my databases have finally completed uploading. It took over 5 hours to upload 6 databases totaling 17.76 GB. My iMac’s hardwired upload / download speeds are currently 859/720. Going to attempt uploading to my iPad.
Mine stopped again. There has been no attempt by DEVONthink to sync for 1 hour and 7 minutes. When I go into the sync tab in properties, all options are greyed out, with the exception of Refresh List. Clicking Refresh List causes the spinning circle to appear over the icon of the iCloud (CloudKit) location, but no other operation takes place. At 1 ½ hours after the last activity I will restart DEVONthink again and see what it does.
