Yes you are right (as I know), it's not possible. The only way to achieve this is to create the VM without a disk, remove the default controller, and then add the desired controller type and create a new disk, the process is something like this.
$vm = New-VM -Name "newVm" -VMHost $vmHost -Datastore $datastore -DiskGB 0 -GuestId ... # (-DiskGB 0) means no disk
$oldAdapter = Invoke-ListVmHardwareAdapterScsi -Vm $vm.Id
Invoke-DeleteVmAdapterHardwareScsi -Vm $vmId -Adapter $oldAdapter
New-ScsiController -VM $vm -Type VirtualLsiLogicSAS
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