How do I solve this: A key cannot be configured on 'Portal' because it is a derived type

I have three entities in my (SQLite or MySQL) database called Identity, User and Portal. Identity is basically a Guid with some extra data, while portal and user are derived classes where portal adds an URL and User adds a username. Basically, a simple structure.
In my database, I want three tables with the Table-Per-Type principle. But I will have only one DbSet as I can get the others with Identity.OfType().
Now some code:

public abstract class BaseIdentityConfiguration<T> : IEntityTypeConfiguration<T> where T : Identity
{
    public virtual void Configure(EntityTypeBuilder<T> builder)
    {
        builder
            .Property(r => r.Id)
            .HasColumnOrder(0)
            .IsRequired()
            .HasColumnName("Key")
            .HasColumnType("varchar(36)")
            .HasComment($"Primary key");
        builder
            .HasKey(r => r.Id)
            .HasName("PX_ID");
    }
}
public class UserEntityConfiguration : BaseIdentityConfiguration<User>
{
    public override void Configure(EntityTypeBuilder<User> builder)
    {
        base.Configure(builder);
        builder
            .Property(r => r.Email)
            .HasColumnName("MailAddress")
            .IsRequired(true)
            .HasMaxLength(128)
            .IsUnicode()
            .HasComment($"Email address");
        builder
            .HasIndex(r => new { r.Name, r.Email })
            .HasDatabaseName("IDX_NAME_EMAIL")
            .IsUnique();
        builder
            .HasIndex(r => r.Email)
            .HasDatabaseName("IDX_MAIL")
            .IsUnique(false);
    }
}

public class PortalEntityConfiguration : BaseIdentityConfiguration<Portal>
{
    public override void Configure(EntityTypeBuilder<Portal> builder)
    {
        base.Configure(builder);
        builder
            .Property(r => r.Url)
            .HasColumnName("URL")
            .IsRequired(true)
            .HasMaxLength(256)
            .IsUnicode()
            .HasComment($"URL");
        builder
            .HasIndex(r => r.Url)
            .HasDatabaseName("IDX_URL")
            .IsUnique(true);
    }
}

I was hoping I could inherit these configuration settings but apparently that's not possible. Unless I remove the base.Configure(builder); part and thus only configure the additional fields... But then I'm likely cause a problem as some developer will take over this project and wonders why I don't call the base class, then adds the base class call and runs into the same error...
For a second project, I only need a database with Portal records, which is a different database altogether. In that project there won't be identities but I want to use the same configuration to build a single Portal table in that project. (Not Portal and Identity but just Portal.) So, I need to know if the model has already configured the key field from Identity or not.
So, my question is simple. Can I determine in BaseIdentityConfiguration.Configure if the property for Id is already configured or not?