I have been very public about not believing PS6 would be a handheld. And I still insist that it is just not physically possible.
However, there is definitely a business case for there to be a PS5 handheld. A big part would require that the screen be smaller than full resolution so as to save power and processing power.
The idea being if you are going to have extended life for the previous gen, you might as well have new hardware to run it on.
Either way, one thing is for sure. Sony will not be making games exclusively for the handheld. They learned the lesson last time. Whatever they make will just run existing libraries.
PS4 can already work as a handheld, people have made it work in custom mods. PS5 might have to be nerfed to do the same unless there are tech we don't know about.