@burnout426 thanks, that Reddit thread helped, most of the data is recovered, except the tabs, but I guess that'll be a matter of playing around with the Session folder, which I backed up as soon as this happened.
Edit: yeah, tabs recovered now too, had to remove newer Tabs_ and Session_ files first.
I can't believe such a big and widely used product messes up something as important as data migration and it's not hotfixed yet as I see it started happening around a week ago.
As for autostart, that doesn't help. The issue is that the installer for some reason has adding autostart entry enabled by default. I worked around it by adding a parameter to the package manager I'm using - chocolatey. But even after that, which worked for a bit, some kind of internal autoupdate of Opera readded the entry... It should be disabled by default in the installer in the 1st place IMO. Because who wants to autostart their browser with system, realistically? It's not good for performance (especially with more tabs than my 30-40), doesn't have reasonable advantages and you can turn it back on with a matter of a click if it's pinned to taskbar.