Sync problems (Finder now crashing / barely responsive)

I'd say I've been having problems with iCloud for a couple of years now. Never ever in that time have all my devices been problem free. It started with my phone not syncing new contacts. Support seemingly fixed it, only for my iPad to then start having the same problem... then I notice that one device (eg Macbook) isn't syncing files... when that gets fixed it just shifts the issue to another device (iMac)..

About 2 months ago I realised my Macbook wasn't syncing properly (again). It took 27 days of support for Apple to actually do something... in between spending days on end on support calls, getting disconnected for daring to not stare at my screen every split second etc... it suddenly relieved and started syncing. Not sure what they did but I'd been told on the previous calls nobody could give me any support until they'd spoke to person X as they needed to know whether he'd done 'something'.

3 weeks ago I completely wiped iMac and reinstalled everything. This device wasn't syncing. Then, after a few support calls, it started syncing.

Now, though, I'm getting all kinds of Finder errors. I've spent four hours this morning just trying to locate a file and move it, because Finder just hangs for ages, causes crashes in any app that tries to open up a Finder window to save/open... And the errors in Console seem to be something to do with icloud or sync. I've looked in Console logs and see 'bird' and other file issues... Note there are TONS of the 'bird' ones, I haven't pasted them all here, this is just a few examples.

Hoping that getting support here might be a lot less painful than through Apple Support Chat and Phone!

Note that I've done two complete reinstalls of this iMac, and have to transfer from previous device. We've done NVRAM resets, Safe Mode boots, reinstalling OSX without wiping, all sorts.