You need someone who supervises the executor.
If you take a freelance architect and tender the project, I assume that the architect and the company awarded the contract are not connected. In other words, I assume that the architect is independent from the construction company.
For me, it does not bring much additional value to involve a fourth party. Because the architect already takes over the supervision. However, in this model, there is no one who supervises the architect himself. But what else needs to be supervised here? The planning? Yes, you can do that. Then, if you want, you can have another independent person visit and give them the architect’s planning to review. In my opinion, though, I do not need an external person watching how the architect supervises the construction company or someone who reviews the same trades as the architect already does. Since, as I said, I consider the architect independent in this regard, I do not find it necessary.
On the other hand, if you work with an architect or general contractor who is connected to the construction company, then you have to question the architect’s independence and an external expert is required.