Hello again, forumfolk!
"By now" really doesn't fit with "you have been ready".
"By now" refers to something completed before now or at the latest completed just now, but "you have been ready" says that you got ready sometime before now and are still ready now. Since your being ready cannot have completed "by now" and also be continuing now, we have a contradiction.
To make sense out of this, we must eliminate either "by now" or "have been".
Case 1. "Hurry up! I think you have been ready." This implies that you are ready, so what I am saying is that you can hurry since you are ready. Possible I guess, but a little strange.
Case 2. "Hurry up! I think you ... ready by now." I think something (now) about the completion no later than now of something that has to do with your readiness. It's obvious that that something is your getting ready. I think that your getting ready ... completed by now. Here something like "should have been" fits because it means you should have already gotten ready by now. I can very well believe you should have been ready even at the same time I know that you aren't ready.
Case 3. "Hurry up! I thought you ... ready by now." In this case, I am no longer talking about what I believe now but about something I thought in the past. "Would be" or "would have been" fits here. If at an earlier time I thought "you will be/will have been ready well before 7:20" but now I see that it is nearly 7:20 and you are not ready, I tell you to hurry up because your not being ready has created a problem with the plans I made under an assumption now known to be false.