I am currently working on a similar script. This what I've come up with for non-DRS clusters and migrations. I have tested the migration of VM's off of the selected host to other hosts in the cluster, and that seems to work. It isn't very smart (ie it doesn't take into account resource utilization or anything), but it does disperse the count of VM's relatively evenly across other hosts in the cluster.
I have not yet tested the migration back to the original host portion yet. But my logic is that if the original array of VM's that was migrated off the host doesn't change, it should be able to reuse that array and migrate them all back to the host after patching.
Also, if you'd like to see the rest of the script, I can share. The basic idea is to use a do/until loop to loop through an array of hosts in the cluster and patch them all one at a time, accommodating for single host clusters, DRS/non-DRS clusters, and a few more features as well.
$ClusterName = Read-Host "Please enter the full name of the cluster you want to update"
$HostArray = Get-Cluster $ClusterName | Get-VMHost
$HostCount = $HostArray.count
$SelectHost = $HostArray
$HostName = $SelectHost.name
# Migrate powered on VM's off of current host
Write-Host "Migrating VM's off of host $HostName to other hosts in cluster $ClusterName." -ForegroundColor Cyan -BackgroundColor Black
"$(Get-Date) - Migrating VM's off of host $HostName to other hosts in cluster $ClusterName." >> $LogFile
$VMArray = $SelectHost | Get-VM | where {$_.powerstate -eq ‘PoweredOn’ -and $_.name -notlike '*vCLS-*'}
$VMCount = $VMArray.count
$OtherHostsArray = $GetCluster | Get-VMHost | where {$_.name -notlike "$HostName"}
$OtherHostsCount = $OtherHostsArray.count
$VMInt = 0
$HostInt = 0
do {
$TargetVM = $VMArray[$VMInt].name
$TargetHost = $OtherHostsArray[$HostInt].name
Write-Host "Migrating $TargetVM to $TargetHost"
$VMArray[$VMInt] | Move-VM -Destination $OtherHostsArray[$HostInt]
$VMInt++
$HostInt++
if ($HostInt -eq $OtherHostsCount){
$HostInt = 0
} else {}
} until ($VMInt -eq $VMCount)
# Confirm that all powered on VM's are migrated off of current host
do {
$VMArrayCount = ($SelectHost | Get-VM | where {$_.powerstate -eq ‘PoweredOn’ -and $_.name -notlike '*vCLS-*'}).count
Write-Host "Confirming that all powered-on VM's are migrated off of $HostName..." -ForegroundColor Cyan -BackgroundColor Black
"$(Get-Date) - Confirming that all powered-on VM's are migrated off of $HostName..." >> $LogFile
Start-Sleep 5
} until ($VMArrayCount -eq "0")
################################### Perform ESXi update steps ###################################
# Migrate powered on VM's back to current host
Write-Host "Migrating VM's back to host $HostName." -ForegroundColor Cyan -BackgroundColor Black
"$(Get-Date) - Migrating VM's back to host $HostName." >> $LogFile
$VMArray | Move-VM -Destination $SelectHost
