Still no beta, I suppose they need to get it working on Catalina as it's 64 bit only.
I think what will come common place is multiboot OS as not everyone can get a 64 bit replacement app for what 32 bit ones they have.
I don't know if there's any extra bays in the newer iMacs to add hard drives and how easy they are to get to, I know the iMac I had, to change the hard drive I had to take the whole thing apart and LCD out to put a bigger drive in it (or change the cpu or clean the cpu fan).
You can partition the drive but separate drives would be better, if you have usb 3.0 you could boot on externals.