“sabbe sankhara dukkha”
I think it means that the Buddha experienced that the ending of all sankhara is also the ending of a burdensome mind. In other words, any sankhara, even vedana and sanna, represent a certain kind of burden. In sannavedayitanirodha there is not even vedana and sanna. This state is not burdensome. It is the highest and most subtle and comfortable abiding (MN31).
There are also sutta’s that describe how jhana in progressive order becomes less burdensome because formations typical for a certain jhana, cease to exist.
Piti may be experiences as a kind of happiness but from the perspective of cessation piti is burden.