from my xp of the old school tolls system is to try to miss the peak hours near bigger cities. I am mostly missing Belgrade and Zagreb could maybe be a problem. My biggest concern on time wasting is the border. Last time i was lucky as it was 5 minutes with 2 cars in front of me, but there were days this year that people were on the border for over 4 hours.
Still not sure when to go. Check in is after 16h, so early morning would make sense, but i like driving at night and not a morning person :)