I have some questions about sexual activity during Ramadan.

If I sleep with my wife after Iftaar, is there a certain timeframe which I have to do Ghusl within?

And: if I have a sexual dream, do I have to do Ghusl before Dhuhr-prayer? If yes, what do I do if I wake up after the Dhuhr prayer has entered, and then realize I’ve had a wet dream?

Always before starting that day fast a person must be in the state of Taharat now doesn’t matter how he became Najis either by intercourse or by wet dream or something else he has to do Ghusl before Azan of Fajr otherwise if didn’t do Ghusl he had to do qadha oh that day fast and has to give Kaffara aslo.
But during the day time while observing the fast of anyone becomes (Mujnib) Najis by wet dream then even if he would do Ghusl few hours later despite being in state of Janabah is Makruh he is allowed to do so.

I have some questions about sexual activity during Ramadan.

If I sleep with my wife after Iftaar, is there a certain timeframe which I have to do Ghusl within?

And: if I have a sexual dream, do I have to do Ghusl before Dhuhr-prayer? If yes, what do I do if I wake up after the Dhuhr prayer has entered, and then realize I’ve had a wet dream?

Yes you must make ghusl janaba before dawn.

In second scenario if you have a wet dream in the day you need to make the ghusl janaba before you make salat whether you woke up before zuhr or after zuhr had entered.