Just an update on this, I received my PPM max yesterday and first thing I did was pop in my 128G SanDisk Extreme Ultra.
I accessed the hidden Android settings (Under About, software version, hit OK 5 times)
I then went to Storage, and selected my Sandisk, the option to format it as “Device Storage” was there, and on selecting Format as device storage the warning that the card would not work in anything else popped up (Like adoptable storage)
I formatted it as device storage, and the last step was to move all data tot he card.
So in short, it looks like it worked!
My PPM now has a total of 140Gb internal storage!!