How to implement a Read-only collection in DDD using Entity Framework?

Question

C++ / C#

Good afternoon, colleagues!
Question about Entity Framework and Read-only implementation of a collection of related elements when using DDD (Domain Driven Design).
For example, there is an entity "department" with a list of positions in this department:

public class Department
{
  public int DepartmentId { get; private set; }
  public string DisplayName { get; private set; }
  
  // Список должностей в отделе
  protected virtual ICollection<Position> _positions { get; set; }
    public IEnumerable<Position> Positions => _positions.Skip(0);
  public static Expression<Func<Department, ICollection<Position>>> PositionsAccessor = f => f._positions;
  
  public void AddPosition(Position position)
  {
    _positions.Add(position);
  }
  
  ....
}

public class Position
{
  public int PositionId { get; private set; }
  public string DisplayName { get; private set; }
  
  public int DepartmentId { get; private set; }
  public virtual Department Department { get; private set; }
  
  ...
}

For mapping you can use:

class DepartmentConfiguration : DbEntityTypeConfiguration<Department>
{
  public DepartmentConfiguration()
  {
    HasKey(x => x.DepartmentId);
    HasMany(Department.PositionsAccessor)
      .WithRequired(x => x.Department)
      .HasForeignKey(x => x.DepartmentId)
      .WillCascadeOnDelete(true);
    
    ....
      
    ToTable("Departments");
  }
}

The implementation of the Read-only list via a static field is taken from here or here .
Everything is great, you can do Include, for example:

_dbContext.Set<Department>()
  .Where(x => x.DisplayName.StartWith("Some name"))
  .Include(Department.PositionsAccessor);

But the question is: How to implement subqueries to these most nested elements. For example, you need to get a list of departments where there are managers.
In this case, the Linq query would be something like this:

_dbContext.Set<Department>()
  .Where(x => x.Positions.Any(p => p.TypeId == Enums.PositionType.Manager)) // Это срока выдает Exception, так как Positions не является навигационным полем (им является _positions).
  .Include(Department.PositionsAccessor);

How to implement a read-only list of related entities, while leaving the ability to make normal linq queries? Perhaps there is some other good approach besides static field?
I would appreciate any help, thanks in advance!