Command keys with function keys are also sometimes employable, e.g., Command-F4.
I use Ctrl-Option, e.g. Ctrl-Option-M for Markdown or Ctrl-Option-R for RTF. Easy to press, easy to remember.
If you want to change a lot of shortcuts try CustomShortcuts.
control q is what I use. I am sure if it is the default in sorter preferences. I use option q for clipping a page to DEVONthink 3. That works in a similar way. If you ‘escape’ the note remains as you left it by the way. As @chrillek sats I really would avoid using a key without a modifier. Almost guaranteed to cause issues.