I don’t see any reason to stop overnight—the drive is easy to do in a single day.
Why are you planning to take the Porozina–Brestova ferry? Are you thinking about doing some sightseeing in Opatija?
If you’re heading to Zagreb, the Merag–Valbiska route is the better option. It’s shorter, the road across Cres to Merag is in better shape than the one to Porozina, and in bad weather the Porozina–Brestova ferry is more exposed to wind.
Get to the ferry port about 30 minutes before departure and you’ll likely make the next boat. There are no reservations—it’s first come, first served. If it’s really busy and not all cars can fit, Jadrolinija usually adds extra sailings.
The drive from Mali Lošinj to Merag takes around 1 hour 15 minutes, the crossing from Merag to Valbiska takes 25 minutes.