AlwaysOn availability group DR failover with Powershell
I think what you are trying to achieve via powershell is for planned manual failover as mentioned in msdn link.  here

A planned manual failover is supported only when the primary replica
and the target secondary replica are running in synchronous-commit
mode and are currently synchronized. A planned manual failover
preserves all the data in the secondary databases that are joined to
the availability group on the target secondary replica

And what you exactly are trying to achieve &quot; failover between async replicas &quot; is more of forced manual failover with/without data loss which can be achieved via TSQL as mentioned in same msdn link. I don't see PS there but may be someone here who have used can answer. I am not sure but may be you can check dbatools if it provides one
Go through the steps as mentioned laster in that article

Manual failover without data loss
Use this method when the primary replica is available, but you need to
temporarily or permanently change the configuration and change the SQL
Server instance that hosts the primary replica. To avoid potential
data loss, before you issue the manual failover, ensure that the
target secondary replica is up to date.