Hi Rolf,
Correct, EVENT_TX is the best way to know when a message was sent, although for received messages you should use extended messaging with the timestamp field enabled instead.
Also, please note that ANT is designed to allow transmissions to occur within different time slots for coexistence, so some jitter is to be expected. If you require messages to be transmitted with a high degree of temporal precision then you may use a TX_only channel instead, understanding this will disable coexistence.
Cheers