Dataverse entity needs two groups with different Column Security Profiles
You can use simple JS.
but this will fail when user is trying to see this information using advanced find, as we can not able to handle show/hide info here.
Best way to implement your requirement is.
1. Create 2 entities. First one will have All the fields that manager needs. Second one will have all the fields which approver need.
2. Now make sure this records own by a service user. And depending upon the role, you can share this records with the Manager and approver. So that you don't have to maintain permissions on record level and field security level.