سفر دلپذیر

It's not 'too' formal, but it is 'refined'. Using it with friends shows you really appreciated the trip. For a very casual vibe, 'Khosh gozasht' is better.

Yes, it's perfect for business trips because it's professional yet warm.

'Khoob' is just 'good'. 'Delpazir' means it was pleasing to your heart and senses—a much stronger and more beautiful word.

Say: 'Safar-e delpaziri ra baraye shoma arezoo mikonam.'

Yes, in standard Persian, the Ezafe is always pronounced, even if it's not written.

Yes! 'Ghaza-ye delpazir' means a delicious and pleasant meal.

Frequently. It's a very 'poetic' adjective that has moved into daily speech.

You could say 'safar-e sakht' (hard trip) or 'safar-e bad' (bad trip), but there isn't a single compound opposite.

No, in Persian, the adjective almost always comes after the noun.

Not at all. It sounds educated and polite, like something a well-traveled person would say.