Handy guide to Inkscape keystrokes, if you’ve never seen:
https://inkscape.org/en/doc/keys.html
Something to remember, sometimes things need to be shifted, so like “union” is control - +. This means you have to hold down control and shift and hit the “=/+” key.
Wow! Have YOU memorized them all!? 
Heeeck no. I still fumble through a lot of menus. I do use the following all the time:
Combine path (cntrl K)
Break apart path (cntrl shift K)
Extend a path along itself (hold down control and alt as you drag a node)
Constrain movement to just X or Y direction (hold control while moving)
Preserve aspect ratio while scaling (hold down control as you drag the scaling handle)
Preserve center location while scaling (hold down shift while scaling)
Union (control +)
cut/copy/paste/undo of course.
Rotate 90 degrees (control-[ and control-])
All of the panning key modifiers
page up/dn for altering z-order of objects.
Other than that I’m pretty much a mouser.
Ctrl-d should be near the top of your list… “Duplicate”, a one-stroke copy-and-paste-in-place.
I agree, should. I’m a control-c, control-alt-v guy. Too many years of control c to roll back.
Necro’ing this thread, because it’s handy.
I now also use these shortcuts a good bit:
rotate by 15 degree increments {[ and ])
to convert object to path (shift-control-c)
convert stroke to path (control-alt-c)
Convert object to guides (shift-G)