Compared to day 2 today was relatively uneventful.
When I checked in the morning, compatibility report appeared saying that everything would be fine:
Other than that I've installed a number of recommended updates, which kept appearing after each run, trying to get Windows 10 to start downloading. To help that I've tried free some more space on the system drive and found around 20Gb of orphaned iPod backups. I've even tried to force Windows to start downloading the Windows 10 update files at no avail.
Still at "We're validating Windows 10 for your PC". Let's wait.