Hi,
>do you have to migrate everything to the Enterprise version or just the replica node?
All replicas in this AG should be enterprise edition.
>What is the point of AlwaysON with the STD version?
The Always On Basic Availability Groups replaces the deprecated Database Mirroring feature and provides a similar level of feature support.
Note that the Standard Edition of SQL Server 2016 and above only supports the Basic availability groups and limited failover cluster instances(Support for 2 nodes) features .More details check this document.
>This platform that I have configured WSCF + alwaysON is not valid for HA?
This topology can provide high availability and disaster recovery.
According to the description in the question, It seems that you have created SQL Server 2017 Failover Cluster Instances on this WSCF cluster before configuring the Always On availability groups. The SQL Server Failover Cluster Instances itself can provide high availability.
Please note that SQL Server Failover Cluster Instances (FCIs) do not support automatic failover by availability groups, so any availability replica that is hosted by an FCI can only be configured for manual failover. Check the following doc for more considerations.
Failover Clustering and Always On Availability Groups (SQL Server)