I have a personal rule. If what follows "instead' is a complete sentence, I include a comma. If it's an incomplete sentence, I don't. I decided this after checking out real examples:
Instead, he sleeps beneath a blanket on a mattress with a built-in pillow shape. Source.
Instead approach the hiring manager as if you were a consultant or collaborator.
(Source of the examples.)
Is this a sound rule? A grammatical one?