Looking at all the various options your best chances may be just renting a car for the duration of your trip.... there used to be a direct flights with easyjet but that's gone :-( Trains are going to be long and expensive... It's a bit of drive but can be easliy done in a day. All train trips seem to take at least 24hrs..... 

Thanks. Martin. Yes, I have a car to drive. Could be an option. May be even cheaper.

We are looking for a crew member. May be we will find one who could join me for the drive.

Thanks again

Jing

You are welcome, went 3 -4 times by car to Istria, works absolutely fine... Just don't forget the Slovenian Autoroute vignette - which now can be ordered online :-) 

Thank you for the info about Vignette. Now I am checking all countries I will drve through this year.

Early September is perfect for Pula, warm weather, fewer crowds, and the sea still great for swimming. I’d also recommend a day trip to Rovinj or the Brijuni Islands if you have time.

Good tip about Rovinj and the Brijuni Islands.I’ve heard both are great spots to explore if you’re based in Pula, and it’s nice to know early September still has good swimming conditions. thanks