I eventually got it to update. I'd had a seemingly endless circle of error 1005/206/210 but following this guide I managed to get it to update using some combination of deleting the patchdata folder, downloading the new launcher, downloading the patchmanifest file and rebooting (I think in that order, but I wasn't keeping note). It was a pain and I don't see why an installer couldn't have done it for me, but it did finally get things going without having to re-download the whole lot, which would've sucked.
I've had this issue a lot too but it stopped when I installed ESO on a new ssd with huge free disk space left. How much disk space do you have left on your ESO drive?