Make the button-mashing to apply/escape submission holds extremely strenuous. Or at least make it an optional selection, eg in the options menu: "escape/apply holds" -> "easy", "medium", or "simulation"
Wouldn't it add to the realism if you tire you arm out button-mashing to sink in a choke?
You could even make a meter gauge to know when you're almost done.
Make some submissions more timing, less mashing (like a triangle choke), and some less timing, more mashing (like a rear naked choke).