I've heard from one Android freelancer developer, that he creates pristine VM for new project, uses it via VPN, creates new Google Account, uses it only inside that VM and does not use that VM for other tasks, so there's no correlation between his accounts.

Technically they're already linked - it's an android, it can query the bluetooth mac address, the wifi mac address, the hostname of the device and the serial number / imei / esn of the device.

There's no "unlinking" it unless you manually modify the serial numbers on the device, which is possible, if you have qualcomm diagnostic tools (QPST/QXDM) and are able to communicate to the modem softband directly.

That's generally true, and the solution is to avoid linking them.

It's a rock-and-a-hard-place problem, since the most common scenario Google is dealing with is criminal enterprises trying to obfuscate their activity behind dozens of "independent" accounts (so of course they have to try unifying accounts). But this is analogous to the scenario one deals with separating concerns when forming an LLC; you keep your work and personal stuff separate, including finances and capital (i.e. don't do work stuff on your personal computer; that's riksing having your LLC status collapse if someone has occasion to sue you).