The new System Monitor does not have a default keyboard shortcut (at least for me on OpenSUSE Leap 15.4 Argon). I would suggest adding Ctrl+Shift+Esc for the new System Monitor. Many people will be familiar with this as it matches Windows. This way Ctrl+Esc can still also be used to access the less resource intensive and older systemmonitor.
Not a bad idea. Feel free to do it.
Note that for new global shortcuts, we are using the Meta/Super/Windows key as at least one of the modifiers, since these are reserved for the shell and global shortcuts, and we don't want to conflict with shortcuts in apps that use only Ctrl, Shift, and Alt.
This has been done for Plasma 6, where the shortcut is Meta+Esc.