Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Bangkok to Sylhet as you normally would.
For a flight from Bangkok to Sylhet, you will be flying into Osmani Intl. Sylhet Osmani Intl (ZYL) is just 6.9 km from the centre vof the city. Osmani Intl is the only Sylhet airport and it handles 0 inbound flights from Bangkok per day. 0 airlines have flights to Sylhet Osmani Intl from Bangkok on a regular basis.