Something i recently learned:
Your live usb does not need to be the same distro as your main. (And in retrospect that makes total sense but i never realized)
My arch install broke and could not get timeshift to roll back using the arch live usb. But a ubuntu-desktop live usb worked flawlessly.
For new users the main question is not what operating system but what window manager as that is what shapes day to day user interaction. KDE plasma is a solid choice.
Installing it on a separate drive should be no problem. Just select the correct drive during install.
I use F10 to get to the boot menu on drive and select the drive it needs to boot from there. I have used it once in the last year and although it required many updates its still working.