I have not run out of polyphony in the past 15 years or so, so it is not a factor for me. There are different ways of allocating the notes regarding priority, so maybe they have gotten better at that as well.
I suppose if you layer multiple voices, leave the sustain on, use busy styles, etc., all at the same time you might run out of notes, but I don't do that.
Solid state drive is in my Korg PA900.
Solid state drive is available for Ketrons.
My USB drives work instantly for what I use them for. I don't interface with PC except to transfer files, and I most generally do that by taking the USB gizmo and plugging it into the computer.
I think the "big 3" in arrangers will have to move on without Roland, at least for now. They have no current top of the line arranger, and the BK9, although very good, has many limitations. If there is a big 3, then Ketron is the third, and smallest of the companies.
If one is primarily interested in home or studio recording, using VSTs, multiple layers, multiple recording tracks, etc., then probably the best way to go would be computer software.