I'm not sure this works properly.

If you simply specify premium economy with no fare class specification, ITA will put you in a cabin that is premium economy or higher on every sector. That means that on the short-hauls, you typically end up in business class.

If you specify cheapest but then use the fare class switch, ITA will properly return itineraries that use the T class fare for all sectors, booking into economy on the short-hauls and T class WT+ on the long-hauls.