Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Bangkok to Bern as you normally would.
If you book a flight to Bern from Bangkok, you will be flying to Belp, which is the city’s only airport. Belp (BRN) is located 5.9 km from the centre of Bern. There are 0 flights from Bangkok to Belp per day from 0 different airlines.
This is defined as Bangkok Suvarnabhumi Airport (BKK) and Belp Airport (BRN). Codes like Belp Airport are often used to just call airports uniquely. While you’re still figuring out your travel plans, it’s not a bad idea to learn the names and codes of other airports around these areas: Bangkok Don Mueang Intl Airport (DMK), Rayong Utapao Rayong Pattaya Airport (UTP), Hua Hin Airport (HHQ).