EF Core - may cause cycles or multiple cascade paths

For others that finds this question, this is enough:

modelBuilder.Entity<User>()
     .HasOne(u => u.Suburb)
     .WithMany(s => s.Users)
     .OnDelete(DeleteBehavior.Restrict);

If you don't have want a list property in Suburb with Users you can do it like this as well:

modelBuilder.Entity<User>()
     .HasOne(u => u.Suburb)
     .WithMany()
     .OnDelete(DeleteBehavior.Restrict);

The error already says what you need to do. Specify what it must do when there is an action. You should add the .OnDelete() method to each foreign key definition.

modelBuilder.Entity<Tenant>()
                .HasOne<User>(s => s.User)
                .WithMany(ta => ta.Tenants)
                .HasForeignKey(u => u.UserId)
                .OnDelete(DeleteBehavior.Restrict);

For further information please read https://www.learnentityframeworkcore.com/configuration/fluent-api/ondelete-method

Your User entity ForeignFey Fluent API configuration should be as follows:

modelBuilder.Entity<User>()
     .HasOne<Suburb>(s => s.Suburb)
     .WithMany(u => u.Users)
     .HasForeignKey(u => u.SuburbId)
     .IsRequired(false);
     .OnDelete(DeleteBehavior.Restrict); // <-- Here it is