Did you try with the VMware.Hv.Helper module?
You can use Set-HVPool, and you probably need to use the desktopSettings.logoffSettings.powerPolicy setting.
Note that the case for the Key is important
Something like this
Set-HVPool -PoolName $name -Key 'desktopSettings.logoffSettings.powerPolicy' -Value 'ALWAYS_POWERED_ON'