This is the basic template for what you asked for (defaults to harm).
If you provide specifics for each combination you’re interested in, I can optimize it further.
CTRL + HELP: CTRL_HELP
HELP: Regrowth
CTRL + BEAR: CTRL_BEAR_HARM
BEAR: Mangle
CTRL + CAT: CTRL_CAT_HARM
CAT: Shred
CTRL: CTRL_HARM
DEFAULT: Wrath
#showtooltip
/cast [mod:ctrl,help] CTRL_HELP; [help] Regrowth; [mod:ctrl,form:1] CTRL_BEAR_HARM; [form:1] Mangle; [mod:ctrl,form:2] CTRL_CAT_HARM; [form:2] Shred; [mod:ctrl] CTRL_HARM; Wrath
You might want to check these out too: