Keeping steaks hot after cooking in a cast-iron pan
If you cook rarer you don't need as long a rest period, for rare and medium-rare you only need a minute or two. Put the steak onto a row of chopsticks or a cooling grate if you aren't using a warmed plate so the plate doesn't cool them, then put a bit curved pot lid over the top. You can use a tent of foil if you need, it's just that there's less waste if you use a lid. 

If you are cooking many steaks and can't serve right away then put the cooked steak into an oven just a bit lower than the temperature you want at the center of the steak, this will keep it warm without overcooking it. Don't do it for too long though, you'll dry the steaks out.