XOSkel10 says:
OOC:
Table ruling question, since you have two characters with steady aim (and I have never had to deal with it at my table). Can I take my steady aim bonus action, hold my action for a triggering event, and still get the benefits of steady aim?
Good question. The text of the bonus action is as follows:
[ +- ] Steady Aim (3rd-level rogue feature)
As a bonus action, you give yourself advantage on your next attack roll on the current turn. You can use this bonus action only if you haven’t moved during this turn, and after you use the bonus action, your speed is 0 until the end of the current turn.
Since the text says it only works for that turn that it is used, I will say that you cannot use it and have advantage on your readied action.