You certainly need to be able to do the thing you're readying at the time you actually do it, but it doesn't have to be something you could have done on your turn. I can ready an attack even if there is no available target on my turn, for example. The other issue is that it's not clear that readying movement actually even involves taking the dash action, since the rule on readying calls out movement as a special case. I mean, you can define your house rule however you want, so it doesn't matter that much, but I'm just pointing out that if you wanted to rule out readied dash contingent on how you spent your bonus action, saying "you can't take the dash action twice in a turn" doesn't do that. You'd have to say "you can't dash twice in a round" and clarify that you interpret readying movement to involve taking the dash action. The only other things I can think of in the rules that are restricted by round, rather than turn, are reactions themselves, and legendary actions. But even those aren't restricted in a way that is contingent on what happens during the turn. So it seems like it'd be not just a new rule, but a new kind of rule.I Readied a 2nd Dash on my turn. Being unable to Dash twice would have prevented this, as you cannot Ready an action you don't have available (a Champion Fighter, frex, cannot Ready a Meteor Storm just because it doesn't hapoen on his turn).